did You find any valid answer for your question since? I'm in the same situation and I'm looking for supported SRM deployment and migration models with embedded PSC for SRM 6.1 and 6.5.
I think if that external psc is really a replication partner then you could repoint. Usually when you setup srm to point to a psc it would ask which vcenter point to (if you have multiple vcenters in the same sso site that the pscs are associated with)
you can check the SRM documentation - section Configuring the Platform Services Controller and Selecting the Correct vCenter Server Instance in an Enhanced Linked Mode Environment
It's recommended to re-point VC to new PSC and after that to modify SRM installation to use new PSC with old VC.
Hope this helps,