The Main Sequence developer confirmed that demo saves can carry over to the full game. That answers the compatibility question. The June reply gives no transfer procedure, so keep the original save until you have verified what the released build can load.
What the developer committed to
A player asked whether a demo playthrough could continue in the paid game. The developer answered yes in the Steam discussion. That reply establishes the plan for demo saves. It gives no basis for assuming that an older test branch follows the same process.
What to do with your current playthrough
Keep the demo save available until you have checked that the full game opens it. If you have a second save, keep it distinct from the one you want to continue so you can tell which world you are loading.
Check the release instructions first
Look for the launch notes and the full game's load options before moving files. If a save is absent or fails to load, record which build made it and which build is trying to open it. Those details help distinguish a missing save from a load error.
See developer confirmation in the Steam discussion for the official game information.
FAQ
Does the developer say the demo save carries over?
Yes. The June Steam reply confirms that demo saves can carry into the full game.
Did the reply specify where to copy save files?
It gave no file path or manual transfer procedure.
Does the statement cover a September playtest save?
The question and reply concern demo saves. They do not establish support for separate playtest builds.