It’s pretty high up the wish list with 8 votes (see https://trello.com/c/TVfkqfZ3), so I hope to make it available this year.
I will go with the solution of different releases of your game. So if you share your game, it will become a separate version. If a player loads a save game, the player will stay in that version, even when there is already a newer version of that game available.
After that I can see how easy and robust I can make it so savegames can be upgraded to the latest version of the game. But the one who made the game might need to take certain things into account so upgrades work fine.
So first I will implement that you share your games to certain releases, and after that I can make the save/load games. This year 🙂