If all of these PSCs are in the same SSO domain, you have no problems. If the PSCs are in separate SSO domains, then you will need to use the script in William Lam's blog.
The topology kb article details the fact that at topology with an embedded PSC and an external PSC is depricated and not recommended. Below are excerpts from the PSC FAQ KB article, which detail that you cannot move a vCenter instance to a different SSO domain, nor can you merge SSO domains.
Can I repoint vCenter Server to other PSCs in the same vSphere Domain? Can I repoint vCenter Server to a new vSphere Domain?
When multiple, external PSCs are deployed in the same vSphere Domain and are replicating, repointing a vCenter Server between these PSCs can be performed vmafd-cli. This allows customers to move vCenter Servers between PSCs in the event they need to performance maintenance on a PSC. For more information, see Repointing the VMware vCenter Server 6.0 between External Platform Services Controllers within a Sit....
Introduced in vSphere 6.0 Update 1, customers can now move their vCenter Servers between different sites within the same vSphere domain using the cmsso-util repoint CLI. For more information, see Repointing the VMware vCenter Server 6.0 between Sites in a vSphere Domain (2131191).
With vSphere 6.0, you can no longer repoint a vCenter Server node to a PSC in a separate vSphere Domain as was available in vSphere 5.5 and 5.1. This is due to the other vSphere Domain not containing any of the important data from the originating vSphere Domain's VMware Directory Service as the two domains have no way of replicating to one another. Due to the way that vCenter Server now stores some data in itself but utilized some data in the vSphere Domain, you must perform a re-installation of vCenter Server if you would like to change domains.
Can I merge two vSphere Domains together?
No, there is no way to merge two vSphere domains together.
VMware Platform Services Controller 6.0 FAQs (2113115) | VMware KB
Regarding Cross-vCenter/Cross-SSO vMotion, this document states that:
- When using the vSphere Web Client, both vCenter Server instances must be in Enhanced Linked Mode and must be in the same vCenter Single Sign-On domain so that the source vCenter Server can authenticate to the destination vCenter Server.
Cross vCenter Server vMotion requirements in VMware vSphere 6.0 (2106952) | VMware KB
The link to William Lam's blog entry provides a script (outside of the vSphere Web Client) to migrate a VM across a vCenter that is in a different SSO domain
