Don't know about remote desktop but the to change the admin account go to System Preferences ->Accounts. Select the account and uncheck the admin box. I think you have to have another admin account first. Also you need unlock it with the lock icon in the bottom left.
If you install the Remote Desktop Client (free) you can still connect to your computer with VNC. There's a guide on how to do this (with some extra stuff about SSH tunneling that you can ignore) somewhere on this forum. Let me find it real quick.