Biggest differences for the original versions to know?

I have played the Final Mixes for 1/2 that are in the PS4 collection.

I came into possession of someones old PS2 games including the original NA releases for KH 1 and 2. Was thinking will give them a play.

I wanted to know what the biggest differences I will encounter from the Final Mixes are though. Dont want to do a weird build or strategy or something that doesnt work here.