Change Usernames

[GerhardK]

Hey,
i hope i've chosen the right forum... and i am sorry if not.

Well my problem is following:
I rent a vServer with ispCP Omega on it. I used one Domain for all the resellers i setup and it was like EXAMPLE.com. Now i gave the Domain back to my provider because i want to use another domain this Domain has the same "name" but another ending like EXAMPLE.eu.
I tried to change the Usernames but it seems impossible trough the gui :S. I also searched some files but i had no success.

so what i want to do is:
transform 'reseller.example.com' to 'reseller.example.eu'

Thank you!