What seeker means is that it will take no small ammount of conversion. There are significant differences in the rules and (probably more importantly) expectations of what an adventure should contain and what constitutes a good encounter.
Once you have some time dming under your belt, you can attempt it but it may be a lot of work (depending on the module). As usual, you can post specific questions about specific modules here to get suggestions though. As long as your thread doesn't deevolve too quickly into an edition war you will probably get something useful out of it.
Why are you using the module again? Because you want to not do the work maybe? Because you want something you know was designed with a known level of content say maybe beginning at 4th and doing through to 10th?
All a module is, is a story and setting with plot twists and encounters designed for a specific level of play. But it is not hard to sort out the plot twists from the encounters. Plot twists take next to no brains though. No one is really going to need to buy a module just for some plot twists. It's the encounters you pay for.
The game though has evolved a lot since 2nd edition. And while 2nd has some great adventure modules, they are all designed to be used with 2nd edition stats of the creatures traps etc taken into account. A 1st level creature in 2nd edition is NOT the same as a 1st level creature in 3rd or 4th and assuming they are 'sort of close' is going to make you very unhappy when you find out the hard way.
Just examine a classic Kobold from 2nd thru 3rd and into 4th. Not the same adversary. Not found in the same numbers and not possessing the same abilities.
Players have vastly differing abilities. A 2nd edition magic using type has nothing in common with one in 4th. In 4th the player doesn't pick spells in advance from a list of choices. And in 4th, a player can cast their at wills as often as the warrior can swing their weapon around. You won't be able to just wait till the 4th edition magic user is out of spells, as it isn't going to happen.
The amount of effort required to modify an old module to be useful in a 4th edition game is sufficient that a DM should be just told 'look drop a few bucks and just treat yourself to buying an actual 4th edition module'.
No one is going to call me anything less than a veteran DM. I have access to every module to have ever seen print. I've done the muse of whether I can 'convert old modules for 4th' and the reality is, there is not enough time in the day for it to be time well spent. Takes as much effort to convert an old module, as it does to completely write from scratch your own creation eh.
Is your day really that long?
I've done the muse of whether I can 'convert old modules for 4th' and the reality is, there is not enough time in the day for it to be time well spent. Takes as much effort to convert an old module, as it does to completely write from scratch your own creation eh.
Note: The above is Tsukino's assessment. Your mileage may vary. Mine does.
I am assuming Tsuki's creativity level is much higher than my own, and that the time it takes him to make up an adventure from scratch is much less than the time it would take me. If this assumption is true (that Tsuki is a creative person who can create adventures fairly easily) then yes, the amount of time it takes to convert an existing module to 4e is about the same as the time to create an adventure from scratch.
I, however, am not an inherently creative person. I use modules because they provide the creativity that I lack. I am an editor by profession. I can edit a document (even a 600-page book, my current project) much more quickly than I can write one. And I can edit an existing adventure module much faster than I could ever write one from scratch. I have even managed to acquire sufficient proficiency as to convert a 2e adventure module (F1 Dwellers of the Forgotten City) on the fly, while I run the adventure.
The main thing I am looking at are combat encounters and traps. I am simply looking at enlarging the combat areas so that everyone has room to manoeuver, adding cool terrain features and switching the monsters to their 4th edition counterparts as much as possible (including adjusting numbers). I will probably have to take more liberties with the monsters at some point because some of them might not match in abilities or level to what the encounter goals are in the original.
I will also look at possible skill challenges in some areas.
This gives me a very cool campaign where all I have to worry about are the encounters. I don't have to come up with a plot, cool NPCs and the rest. This saves me time and gives me the possibility of working more on the encounters if I want to, to make them even better.
1. Convert the monsters of your 2e adventure using Adventure tools or replace them with others from your MM or MV book
2. Change the damage for the traps - if any (I use DM's kit to find the right damage fom my group's level while keeping the original "mechanics" of the trap)
3. Change the amount of gold found (again DM's kit - not sure which DMG you could use as I don't own any)
4. Replace the scrolls/magic items the PCs find in your 2e adventure with anything that could be usefull to them (I use potions of healing or other minor items most of the time)
5. Imagine a way to replace something that your players don't have (if the 2e wizard has to use "speak with animals" at some point but your player doesn't have something similar, then change the story or use some skill challenges). My favourite is to create a NPC that travels with the PCs for some time, or an imprisoned NPC found in the dungeon, someone that could give your players the extra information needed.
6. Be creative. When your players do something that doesn't "fit" the story, just invent a workaround. But that's something that you have to do even with 4e modules...
My conversion was an equal amount of joy and frustration. The monster and encounter tools are invaluable. The actual conversion can be thorny. The 2e, 4th level monster, key to the story, is now a 17 level critter with none of the abilities or social roles the 2e creature had. Hmmmmm. Story now doesn't translate into a 4e realm. So, some things work, some don't. Sometimes the things that don't work end up being big things. I invested the time to build lots of custom monsters. Neat that you can do that? Yes. Worth the time? Not really. Especially not for a "conversion." But I'm glad I did it - my personal 4e baptism of fire. Still, there was no practical reason for it.
If you feel your own "from scratch" creativity is below your personal standards, some forum posters might take general ideas and flesh-out those for you as a simple, enjoyable exercise. I would only say that you state your intentions upfront. If you want to go into business and are looking for free intellectual labor using some "I'm a helpless uncreative DM" ruse, well, that would make you a jerk. If you are playing D&D with your teenage kids and want to knock out a few months of original game playing for them but can't clear the "I need a story line" creative hurdle, I'd bet there are plenty of people who would share or generate ideas under the DM forums.
Good luck.
I updated a personal 2e adventure for 4e for no particularly reason other than being headstrong, or maybe as a rite of passage to my acceptance of 4e.
+ nostalgia... I haven't played since 1998 (and I have never used the 3e rules), so when I play one of my 2e modules it reminds me of when I was a kid 
The 2e, 4th level monster, key to the story, is now a 17 level critter with none of the abilities or social roles the 2e creature had.
I do direct substitution when I can, but when faced with what you had, I grab a level-appropriate monster (usually matching conceptual role more than anything else) and use it as is mechanically, describing the creature the way I want it to be. I do this on the fly, rather than spending the time creating a new custom creature on paper or with the monster builder. The new creature does the exact same attack and damage stats as the one I'm "using", with everything simply described the way my original monster looks, acts, etc.
In this one instance, nostalgia was a driving force. There was also a social aspect of the specific creatures that affected the geography, the buildings, why they would be there, what the "village" had witnessed, what needed to be resolved, etc... and they served as a hook to another adventure because of there social involvements with more wicked creatures. Custom making creatures was fun, and since I had an old monster manual to base the creatures from, they came out fairly balanced, I think. We'll see. I'm going to run it as soon as the VT beta comes back on-line. Maybe it came out horribly. But even with a good source - creating monsters can consume hours and not every ability from 2e exists in 4e, so you have to replace those abilities with something that fits and is available in the 4e attack/effects palette.
My experience was that, one time adventures could be converted to 4e, but significant work is involved. Some don't translate well at all. Mine didn't without ridiculous effort. It consummed hours of times to get the creature encounters converted. It would have been faster to change the storyline to fit different, but standard 4e creatures.
+ nostalgia... I haven't played since 1998 (and I have never used the 3e rules), so when I play one of my 2e modules it reminds me of when I was a kid
If you haven't played since '98, jump back in. My feeling about 4e (play and love whatever version you want) is it works and I treat it like I did when I first started playing - it's a game and I enjoy with family & friends, and I meet great people through it. While I could criticize weaknesses I'm apt to take a tolerant approach, but I do applaud those who feel compelled to push for improvements. I sort of enjoy the flaws in the game the way I enjoy the flaws in my PC's. Give me a low stat roll, I can play the character regardless.
I took much of the 80's and 90's off of D&D, but I did play some sporadically in the 90's with my kids. Getting back in the past couple of years has been very enjoyable. I must warn you though, if anyone criticizes 4e with the phrase "It isn't your father's D&D" - my overly short, overly heavy (bizarre roll) dwarf we call "The Wall" will cut them at the knee caps.