Changing my accounts name?

[ubiquitous]

Sorry this has been already posted, I used search function and couldn't find anywhere so I made a post.

My account name I used to try the demo is guest<random numbers>blahblah

I got to level 20something before my demo on the account expired. Now I want to buy the game but I am wondering if I can use that guest to buy the game with and continue on with my leveling from 20s but also to change the guest name to something more close to my current name

ty in advance

[Karl]

Login with your guest account there will be a button that says enter key. When you try to register a key to your guest account it will force you to enter a username, nickname and password.