I made a bulk username to UUID converter tonight out of boredom. It's not very developed (and is probably full of bugs, since I made it around 12:15 AM), but I thought it'd be worth sharing. https://github.com/forairan/ConvertUUID It won't be much use to you right now unless you know how to compile it. Once it's a bit more developed, I'll share a precompiled binary. This may be useful in the future, since as of Minecraft 1.7, name changes may become possible in the near future.