When you create a subscription, the user name can be freely edited. You may need to open one of the optional drawers/fly-outs at the bottom of the subscription-creation-dialogue.
The user name can also be changed by the "admin" afterwards. Best practice is to
1) suspend the subscription (this is needed to keep running processes from blocking a user name change)
2) enter the "webhosting settings" dialogue page and change the user name.
3) enable the subscription again.