Hello,
The MoveClient utility will do this.
It's in the NoSupport directory under the Tools folder in the 14 download.
https://support.symantec.com/us/en/article.tech157429.html
You cannot move clients to another group by using sylink.xml if they have already been registed with the SEPM. You can indivudally move clients by right-clicking on the client and choosing move and selecting the appropiate group.
NOTE: you can select multiple clients as described in the article below.
Moving a client computer to another group
http://www.symantec.com/docs/HOWTO80799
Another option is to use the web services API to move the clients. Documentation can be found in the following locations:
SEP 12.1.x
\Tools\Integration\SEPM_WebService_SDK
SEP 14
\Tools\WebServicesDocumentation