Thanks! Happy New Year 262177!0.19 already? You're pretty fast. Happy New Year Alvir.
About the structural changes making the saves incompatible, you'll probably run into that a lot until you find a satisfactory final structure for the data since it's still quite early. To prevent frustration from players who grind for hundreds of game days (probably not the majority but since it's a training sim, maybe a few at least) there are multiple approaches I can suggest, ready-to-play saves from your own playtesting or a save structure updater.
The former simply allows players to use your own saves but can be extended into reassigning the variables, objects and classes with a proper structure for saves that no longer work. The latter is slightly more complicated since you have to backup the player's savedata, catch inconsistencies and rebuild the data to match the new structure before updating saves and persistent data (always backup persistent data, especially before doing anything to it).
As Laughingfox said, best of luck in the new year and keep up the good work
If you had a headache above and in case you don't already have it bookmarked:You must be registered to see the links.
Don't be afraid to ask onYou must be registered to see the linksorYou must be registered to see the linkson IRC either, fellow devs would be more than happy to help.
I'll look into these methods, thank you for your detailed suggestions!
Yep, I'm lurking Lemma Soft forums and Renpy documentation, I've found a lot of help there ^_^
Many plans for this new year, new events, polishing of existing mechanics, many new images to replace paceholders~
Thank you for your encouragement!
Wish you in return good fortune and lots of luck in this new year!