The reason why 358/2 Days was not given a full re-release is due to the fact that since it would have taken more time to be able to fully adapt it, because of the Nintendo DS system having inferior graphics compared to the later 3DS model and the systems produced by Sony. This would have delayed this games' release as it would have required to recreate all the graphics from 358/2 Days from scratch to be on par with the other games in this set which is why it was changed into a cinematic movie instead. However, most of the Roxa's Diary entries and Secret Reports from the original game were used in HD version as well.
Since the release of original Kingdom Hearts (2002) the original source code of the game had been lost which meant developers had to recreate the code by extracting data from retail copies of the game.
For the HD re-release of 358/2 Days, new audio based cutscenes were created by remaking scenes that were originally silent in the original DS version with only worded dialogue, such as numerous scenes of Roxas, Xion, and Axel at the clock tower in Twilight Town other than re-using the ones from the original release. These are also notably the only newly recorded voiced cutscenes in this entire collected edition game, as the others are archived from their respective games' original release.
In the original release of Kingdom Hearts Re: Chain of Memories (2007) completing both Sora and Riku's storylines or for Japanese players only, have a save file from a completed game on Kingdom Hearts II: Final Mix+ (2007) was necessary for Sora to get ahold of additional attack and enemy cards inaccessible through other methods. For this new release, these have been removed, and completing the HD version of 358/2 Days and getting all the trophies is the new requirement.
The number of different in-game music tracks from all three games, mostly from Kingdom Hearts Final Mix were re-recorded and re-orchestrated to create more high-quality music for the HD Collection.