Spell Slots change into magic points:
1st level = 1 mp each
2nd level = 2 mp each
...
9th level = 9 mp each
Spell level = mp cost.
Casting at a higher level = add 1 mp per level you wish to boost the spell.
(There is a more complicated version of this in Unearthed Arcana for 3e; I want it simpler.) Here, You can cast more high-level spells or more low-level spells, each at the cost of the other, but I don't think this is a problem, and if I recalculate hp all the time I heal and take damage, I can do so for mp when I cast a spell.
Pros and cons? Anyone tried something similar?
I did something very simular to this in AD&D 2nd edition. You would need to account for 0 level spells, and i always gave the players a set amount modified by their Intelligence at 1st level.
Thanks fo replying! 
I was using this for 5e, where 0-levels spells are at-will, so it can be handled that way too.
Wouldn't this throw off the delicate (some would say non-existant) balance of power among sorcerers, warlocks and wizards? First thing that comes to mind, warlocks are meant to have only a few spells, but able to cast them at high levels. Converting their spell slots to MPs would give them lots and lots of spell options, and versatility isn't supposed to be their strong suit.
Curious to know how its playing out for your table.