According to Nomura, Square Enix decided to make and complete the HD version of Kingdom Hearts prior to the HD PS3/PSVita version of Final Fantasy X, and while this collection includes HD versions of Kingdom Hearts and Kingdom Hearts Re: Chain of Memories, it will also contain HD version of all the event scenes from Kingdom Hearts 358/2 Days, which is around 2 to 3 hours of full HD videos from KH 358/2 Days.The remix however doesn’t have any secret rooms or new additions, other than HD and Trophy support, and a slightly modified battle system for Kingdom Hearts which would be more reminiscent of Kingdom Hearts 2.
But apparently according to reports from this interview, Nomura revealed that they did initially plan for KH 358/2 Days to be remade for the PS3 in HD fashion, but due to team, time, and budget restrictions, they couldn’t create the game from scratch in time for the deadline, so they just converted all the event scenes in HD as a sort of movie style interlude.
Tetsuya Nomura was apparently was asked whether or not there would be a HD remake of Kingdom Hearts 2, and those subsequent games. To which he replied:
“It would be unnatural if there wasn’t one,”
Nomura, of course, didn’t specify if/when this said remake would officially happen, but the remark does lead one to belive it is actually in the works.