Define what you mean with migrate? Just switching to the new master server server? If so, that isn't that difficult, as long as your name resolution is already in place for both source and target domains i.e. everything can resolve and connect to everything (Master, Media, and clients).
Please explain to us what exactly you want to move regarding ' moving the clients'.
About "collapsing NBU domains, ... migrating the catalogs" it seems that you want to merge various NBU domains/master servers, right?
If so, you need to contact a Veritas Consulting Partner.
Merging NBU masters will merge and move the catalogs as well as the clients.
DOCUMENTATION: How to migrate the NetBackup catalog information from one platform to another (UNIX to Windows or vice versa), rename a Master Server, cluster an existing Master Server, or merge multiple NetBackup domains.