Removing domain controller

Had a question on removing the very first domain controller setup in the forest. I have 3 separate stores setup in the forest. One domain controller per location. The very first store has the very first domain controller that was setup in that forest. We are closing that store, and I was wondering what I would need to do in order to make one of the other domain controllers the ???top level??? controller in the forest. Or if I need to do anything at all? We have an exchange server running at store 2, but other than that its pretty much simple setup. Each of those 3 domain controllers are setup as dhcp and dns servers for the individual stores they are installed at. Any help would be appreciated, thanks.