We can only change names to unoccupied name slots. If there is already a taken name then it must be of a certain age/post/pm count before we can change it as you are essentially taking someone's old account. The mods however cannot do this, only Mahz as it does require work to be done to the database.
The reason for this is your old name URL will still work (eg:
Grimhildr will still lead to Grim's account), and if we were to assign you to a previously taken username, there could be database problems with the link back from the old name leading to your new name, or the recycled account (Eg2: Grim wanted
Grimm initially, but that account was taken, held 22 posts, and had a dozen PMs).
If you're unsure of a name, ask and we will let you know what we can do.
Hope this helps.
@WelcmeToGudBrgr