Can I switch a regular game to Adult if no one but me the GM is in it? Or should I delete it and start fresh?

If there are no other players, then there's no problem with switching it to Adult.

My understanding is that you are able to, even if you have players.  It is a relatively simple process, and something we almost did in a game I'm playing in.  First let everyone know that you're going to be switching the game over to Adult, and that they will have to submit a new RTJ once they are removed from the game.  Next, switch all characters over to yourself, and in the process, all other players will be removed from the game.  Once all players are removed from the game, switch the game to Adult.  As players re-RTJ check to make sure they have the proper age statement and then add them back and transfer their original character back to them.  Easy as that!

@LB: That's a nice little tidbit that should be added to the FAQ if it isn't there already. Especially the saving of characters bit.

If you remove the players from the game directly, rather than by taking their characters off them, you will automatically be given the option to switch their characters over to you.