Hello enrich,
Thank you for posting in Symantec Connect.
I would be glad to answer your question.
To move clients from one group to another group is actually a manual process.
If having large number of clients in the group and want to automate the process then take the help of MoveClients utility.
MoveClient.vbs is a Visual Basic script which, when properly configured, will move one or more clients from a SEPM group to another group of your choice based on the hostname, username, IP address or operating system of the client. It also has the ability to switch client mode from user mode to computer mode or visa versa.
The MoveClients utility is unsupported. Need to refer to MoveClients.pdf for full documentation.
If SEPM is AD integrated then will have to move the clients on the AD server itself.