While the dev described as "changing the engine", I think, it’s actually just cleaning up code. It was changed from Ren’py (bad code) to Ren’py (probably better code).
It is very much unclear why almost all content has been removed during the multi-year fixing of code, and why the dev thinks that re-implementing the very same content that had already been there should take another few years.
Seriously, at this rate it will take just as long to add the same, already fully written/drawn/animated scenes back into the game as it would take to develop a completely new game. I’ve never seen the like before, and I’m fairly certain the dev only released an "update" at all because the fuck95 mods decided to tag his game as abandoned despite his assurances that he was definitely still working on it.