What is new in this edtion of the game?

[Potches]

Sorry, Squareenix has a very convoluted way of naming their re-releasesremakes. What is different in this than the old PS version I played ?

[mdesaleah]

Define “old”?

The original PS2 version? Then this is a collection that includes the Final Mix updates that used to be Japan only. Final Mix is the same game just with more, going over everything would take too long

PS3 Collection: this collection added the Final Mix versions for the first time and is what all later ports are based on.

The PC version in particular is the PS4 version of the collection which combined two separate collections into one and added 60fps

In this one you get 1, 2, and the games in between and some after (although two of them are movies instead of games).

2.8 gets you the rest of the games you need before playing 3.

[[pGA]Marie★]

The only new in this collection is the so called AI Upscale textures.

[Terra]

If you played the original PS2 releases, the Final Mix versions of KH1 have some additional cutscenes and an additional superboss (who you probably recognize if you played KH2). It also includes the Japanese only PS2 remake of Chain of Memories, if you were like me and didn't buy a GBA sequel to a PS2 game and were confused when KH2 started. KH2 Final Mix has a TON of extra superbosses and an additional Drive Form. I know the least about BBS Final Mix, but I think it has additional superbosses and an epilogue?

[PKblaze]

Final Mixes are like a directors cut with added enemies, abilities and adjustments.
The 1.5 and 2.5 collection overall is games that released in the order of 1, CoM, 2 and BBS as well as the cutscenes of 358/2 and Recoded.