cPanel & mapping after migrating to WPMU

I'm in the last stages of migrating my three stand alone WP sites to subdomain blogs in a new WPMU network set up for this purpose. For background the new WPMU site and the old sites were all on the same host with no change in IP.

I didn't activate the WPMUDev mapping plug-in yet because I had a ticket in with my excellent hosting service for fear I'd break something they were already working to configure from their end.

But my old domain names are still resolving to the old blogs, not the new WPMU sub domains.

My hosting service says I need to deactivate the cPanel accounts on my old domains then activate the WPMUdev mapping plug-in.

In effect I go from 4 cPanel accounts, one new and three old, to just the one for the new domain which where WPMU resides.

Is this how cPanel works in a WPMU environment? I wasn't aware the existence of a cPanel account itself affected how the URL's resolved.

After that, I should be able to configure and activate the mapping plug-in and my old domain names will transparently display their respective sub domain blog?

Is all this correct?