If you registered a username and you wish to change it that is imposible and also remember that the usage of multiple usernames is forbidden on undernet. What you need to know is that a username expires from the cservice data base when it touches 60 days of inactivity, meaning that you don't login to it on irc for that amount of time.
You can try to self-suspend your username if you launch 2 connections to undernet and login to your username on both of the servers and also issue the self-suspend command that is:
This feature hasn't been created for this purpose but its the best solution for you at this moment after which you can try to register a username and of course, try to make up your mind about it.
If you need further assistance regarding usernames you can join the channel #usernames on irc