Why you would want to change your domain name is beyond me, but there are always legitimate reasons. If you just want to change from .local to .com, I recommend that you look into your reasons. If you need to change the name for political reasons, so be it. A .com name may be used, but be careful. You really don't want to use your company's domain name. Use a sub-domain. Or, register another domain name and only use it for internal naming purposes.
That being said, here's the process. You don't want to join the new server to the old domain. When you do that, you lose the ability to chose a new domain name. Setup the new server using the domain name you desire. When that's done, install ADMT on either server (you can install it on SBS2008 if that's what you're using, but it's a pain). Use ADMT to migrate all user and computer accounts over to the new 2008 domain.
It's a lot more work than a computer rename. You must figure out if it's really worth it. It may be more trouble than it's worth.