heya. so the answer to that is sorta complicated and i'm not sure if you're playing it on pc, mac, or android. So in general, your old save files should be pulled into each new version and those old saves should load fine and work if you want to replay a specific scene or something. however, if you keep playing the game forward from that point, any new material that wasn't in the previous version could (and likely will )get screwed up (either resulting in an error or just blindly skipping parts of the story without telling you). the only real exception to this right now is on the android port which i had to recreate from scratch for v18 so it's using an entirely new key now and that broke everything saved before it. hopefully, that wont be an issue in the future and I'm hoping it will pull previous saves to newer versions.
right now, each new version requires a full download. there's probably a way to create an actual update program but that's beyond my skill level. i'm a psych major dude, i've literally googled every step of this process from scratch and still have no idea what i'm doing 😆
the reason that old saves dont work in the newer version is because the choices in the game establish variables in the coding that end up being used later. those variables have to be declared at the beginning of the game so if i add any new variables in an update, it will break the game moving forward with anything related to that new choice because it wasn't declared at the beginning. Now... with the revamp, i pulled all the old variables into the game from the original so, in theory, it shouldn't break that often. however, in the course of the writing, sometimes i've changed things or cut parts or reorganized things and that required a new variable that wasn't in the original.
so basically some new updates may actually let old saves work while others will crash it. because i'm amateur as hell at this, i only work on one section at a time so i dont really plan ahead for things like that the way that a skilled coder would be able to. I did create the chapter select thing for a way to help speed up this process if you want to recreate a save. you can just jump to the chapter you want from New Game and then if you hold down CTRL, you can fast forward through shit suuuuper quick. i know it's not ideal but this thing is also still very much in alpha as a whole and i'm still just stumbling through it 😏