File structures change as new content is added. y.v. could spend his time coding conversion programs but writes new content instead. I've been beta testing through many versions and, later today, will restart again because content changed. I like more content. You still have the option of starting on later days which was implemented to address your complaint.-
There are ways around this. Essentially you can put in checks for missing variables to 'fill in' anything that's missing in new versions.
default new_variable = 0
will add 'new_variable' (insert variable name here) into existing games, and when the game is started/reloaded, if the variable isn't present, it is immediately added with a value of whatever you assign to the default (in this case, 0)
Classes gets a bit more complex, but I've been able to work around this in various versions of my mod (again, need to add checks to 'fill in' the missing stuff), so that people can use EXISTING SAVES when playing my mod.
My point here is that it's not impossible, as I've been able to figure this out in several different games that I've modded over the years (not just Space Journey)... including IN my Space Journey mod which changes a number of things...
It just comes down to if you are willing to put in the effort to make your players happier/save them some grief...