So basically the way that it works is based on variables. when you make a decision, it either makes something true or false and later on, when the game needs to reference that choice, it uses those true/false variables. in order for the game to recognize a variable exists, it needs to be declared when the game starts-- not on boot-- but when you click new game. Meaning, that when i launched update 1, I would have had to include every possible variable for choice into the game then to never have to force someone to restart a new game. I didn't because i didn't exactly have the story planned out aaaaaand, in the early days, people used to break into the code and try to find variables to figure out where the story was going. To prevent that, i just didn't include any future stuff in the code at all and only planned one update at a time. Meaning when i write the next update, any new variables or choices in the game would have not been loaded at the beginning of the previous game version when you started playing so the game would crash when it gets to that choice and tell you the variable doesn't exist.
to get around that, it's easier to delete old versions and just install the newest version. it usually wont let you play the full story loading an old game but your old game save files would mostly still work if you wanted to replay a certain scene (because all the old variables are still included in the new updates). however, to play the newest update, you'd always want to click New Game and then follow the red prompts. this loads all the necessary variables into the new game including the new ones then asks you some questions to mimic the choices you made before and basically quick creates your old save file with the necessary choices and then drops you at the end of the last update with all the new variables loaded.
the last thing here is that the Revamp has the majority of variables preloaded. i just copied the old variable list from the original game and all of those were loaded at the start of the revamp. however, when i include new dialogue or content, I have created new variables or have renamed things to make them easier to find in my code. So yeah, the old variables from the original game mostly still work in the revamp but not always