Normally, HD remixes don’t seem like a particularly demanding ordeal. The developers just compile a few games from the series in question and ratchet up the resolution on the in-game models. Maybe they’ll improve some textures here or there. It’s work, definitely, but it pales in comparison to constructing entirely new assets for a brand new game.
Unless the game in question is Kingdom Hearts HD 1.5 Remix, in which case the original data for the title was lost. According to Tetsuya Nomura, everything had to be recreated by researching the original titles and digging into them to retrieve what they could. Further, when everything was recreated, it had to be done in high definition.
That’s a lot of work to go through for any game, but hopefully that effort and care shows in the final product, which includes both Kingdom Hearts and Re:Chain of Memories. Kingdom Hearts HD 1.5 Remix will be available September 10, exclusively on the PlayStation 3.
Source: Siliconera