are the stats different, in leafgreen/firered from R/B/Y?
what should i know about the change (if any)?
The "Special" stat is split into two separate stats: "Special Attack" and "Special Defense" which of course means there are two separate, varying stats that affect how much damage you take from a special attack, and how much you deal with one.
Along with DVs/IVs which carried over form RBY, there's an additional stat factor called EVs (Effort Values) which I'm too lazy to give you an overview of, much less find a guide that goes in depth with it.
Also, Base Speed is no longer a determining factor in critical hits.
Freeze is no longer permanent until Haze/Fire-type move/Ice Heal is used, though you can still use the latter two to cure it. Otherwise, you'll thaw out eventually after a number of turns.
Seismic Toss does not affect Ghosts without using Foresight/Odor Sleuth.
Blizzard's accuracy has dropped by 20 points.
Snorlax is still a beast.
Hyper Beam's recharge isn't negated by a KO anymore.
Toxic is actually useful now.
Substitute negates opposing status moves.
After waking up from Sleep status, you attack on the turn you wake up on.
Fire types resist Ice-type moves.
I'm looking at a type match up chart for RBY right now, and Bug is no longer super-effective against poison-types if this chart is right.
That's all I can think of.