Say i buy a hosting package and a domain that comes free with it, call it domain "A" then weeks later i transfer 3 other domains ("B", "C" and "D") over to the same registrar and host as "A".

so all 4 domains under the same host and registrar but the main site and physical space is assigned to "A", how hard is it change it so the main site and all the webspace is assigned to say "C"?

so instead of the index page being www."A".com/index.php its www."C".com/index.php

Is it just a matter of changing the DNS settings of "C" to match what's currently set as "A"?

or is it a bit more involved than that?