I have a multisite set up as well as a regular network

I have a multisite set up as well as a regular network setup with client exec.
The reason for this is some of my clients require their own unique plugins and access .
All my accounts start out in the free trial on my MU.
When the need arises for a move over to my network on clientexec on a subdomain ...what is the best way to move their site while keeping the same subdomain name, theme etc, core plugins etc.?