Creating a local admin for the original workgroup

OK. Hopefully, I can explain this where it is clear. Using WIN7. When it was first set up, it had users, most of which did not have admin status. It was then joined to a domain, so that it was used with users, usesrs with local admin privileges and the domain admin. This is on SBS 2003.

My question is I need to change the name, so I have to delete the computer name on the server, add a computer with the name I want and then rejoin the client computer. This means changing it to a workgroup. When it reboots afterward and is now a workgroup computer ready to be joined using http://computerconnect, there is a logon screen. I am afraid that I may not be aware of an account with a username and password which is a local admin for the original workgroup computer.

Is there a way when I am in the computer while it is still on the domain, to go to User Accounts and set up a user where I know the username and password so I can be sure I am able to log in after the change to the workgroup?

Thanks.

Bert