Turning a free game into a paid game is a quick way to upset a lot of people. I've seen too many good games die that way to recommend it.
A better idea might be a paid Beta - paying users get access to the latest version, whilst free users get both less frequent updates, and are always at least a few versions behind the paid version.
People can be surprisingly impatient, and willing to pay for getting something quicker. Also, players who actually enjoy the game will be more willing just to help - and will literally pay for the chance to help. Please, at least try it.
Good Luck, Autonaughts!