You don't need to buy anything. You can wait until the game is finished or download old updates that is available for free.
Viewing post in The Higher Society V0.16 comments
Either way, you should probably stop taking payments for the old versions when you release the new ones, or at least change the description to say "This is NOT the latest version. Go __here__ to get the latest version" at the top.
After all, anybody who buys this version today for ten cents more than v0.18 is probably going to feel cheated. :-)