Changing the main site URL

I'm planning to change my WPMU install's URL, which will impact all the user blogs that have been created so far.

I see that I can go in to the "Domain Mapping" tool for the main blog and simply add a new URL, but I don't think that's the best way, since it's the main site URL.

1. Is there a list of best practices for doing this? I'm assuming I can login to phpMyAdmin and make a sweeping "find-and-replace" change.

site1.com → site2.com

2. Is there a way to keep the existing site URL and simply add another so that they're interchangeable?

blog1.site1.com
blog1.site2.com
→ both URLs should resolve to the same place