If you are reseller, enter the billing, reseller section (enter as reseller).
Select the user you need, detach him from your reseller cabinet and enter the billing under his login-password.
Change the server for him, and attach him again. The user shall be disconnected from the server.
After you change server, you need to change the server in the user settings (i.e. visit him and make all necessary changes in his receiver). You are reseller, don't forget that you are fully responsible for your users.