Export one multisite into another multisite

I just set up a new multisite with subdomain for domain mapping under brandmerce.com. Now I need to move the websites from my previous multisite sitemerce.com that was set up as subdirectories to the new multisite. They both reside on the same vps server, the old one is under one ip address and the new one is on another dedicated ip address.

After I transfer sitemerce.com (WPMU subdirectory) to brandmerce.com (WPMU subdomain) I would like to erase the sitemerce account and reinstall as a WPMU subdomain and transfer everything back under that domain. Thanks.