I wanna play the full game but I don't have any money right now
The game isn't Full right now, is in the state of Early Access, even though it will be complete as time goes.
No, that old demo is incompatible. There is so many additions in the game that forcing an old save will cause a lot of glitches to appear.
It's recommended to start again, specially because the new Demo/Early Access features a lot of improvements.
