Offline servers and UUIDs

To start: Please, mods, for the love of God, do not lock this. I'm not running an offline server myself; I just want to best serve the interests of the people who use my plugins.

Now that that's out of the way: I'm currently in the process of converting an economy plugin of mine to use UUIDs. Until now, it's kept track of everything by storing the usernames of players. However, since that system's now unreliable, I need to convert existing data over to the new one. While this normally wouldn't present a problem, I'm not quite sure how to handle usernames without a UUID. The only thing I could think of would be to just remove such entries from the table, but I'm willing to bet that would annoy people quite a bit. So: alternatives, anyone?

Unrelated: Is it necessary to use UUIDs if they would only be stored for one session (i.e. until the player logs off), or is it safe to stick with usernames?

 

If you are storing things for only once session, it isn't necessary to use UUIDs, since players couldn't join with someone else's name and steal their money.

Correct me if I am wrong, but I am pretty sure using playernames would be reliable for single session data storage.

 

Alright, wasn't completely sure. I would assume that username updates would take effect upon rejoining a server, but I wasn't positive.