Sadly yes. The Johnson update is largely about moving the last bit of old code to the new code, which will make it all mesh together.
But why the hell? Milfy City has a similar sandbox-like gameplay and the old saves works just fine.
I would ask why Save Compatibility will only be added if people vote for Erik's, but Laziness answered that.
As a modder, i used to decypher both games and i ca assure you that Summertime Saga use very deeper coding system.
It's very smart, need a huge amount of time to do it nicely and it's very well done.
Summertime already have a "restore old save" system, but only for the persistent date saved, like cookiejar unlocks. The data saved is actually more like a memory screenshot written into a text. Even the backward functions works with a summertime save.
So if you want to keep save from version to another, you need to change the behaviour of a save before, choosing data carefully, the way, the how, the why, before writing them and load them. Then, you'll need to keep a specific track about changes between two versions, and write missing data from an old version save to a new version one.
BUT, as i started modding this new mechanics, i could provide a "let me go thru all story until that moment" for each character soon. I'm writing a debug menu from the developper one (getting all safe functions from the dev console) and saw about theses mechanics. I could trigger manually all the events checkpoints until a chosen moment for a specific character, but it could be tricky as many events are dependant of others.
Check my mod thread, i'll keep you updated there.