***Please test below steps before you do it in production. Create test vCenter, PSC servers same as you have in prod and test it otherwise if you miss any steps, you can break your prod vCenter.
From Above document -
Repoint an External Deployment to an Alternate Platform Services Controller in the Same Site (Intra-Site)
Now, what if we wanted to repoint this vCenter Server instance to use an alternate external Platform Services Controller in the same site? For example, to quickly recover from an external platform services controller failure and/or distribute load to alternate nodes that are in the same SSO domain.
Once repoint is successful, Power off existing/old PSC.
Just consider as your PSC servers are failed and perform above steps to repoint vCenter servers to new PSC.
What happens when the PSC 6.0 server is down? How does this affect Enhanced Linked Mode (ELM)?
If the PSC 6.0 server is down, you cannot log in to vCenter Server or any second party VMware products that depends on it. Existing connections and user sessions to the vCenter Server remains active, and the vCenter Server services remains up and running. However, once the session ends, if the PSC is still down, the user cannot log in again. Additionally, if the PSC is down and the vCenter Server's services are restarted, vCenter Server is unable to fully start until the PSC's services are restored or the vCenter Server is repointed to an operation PSC in the same vSphere Domain.
Regarding an environment in which multiple PSCs are in the same vSphere Domain and Enhanced Link Mode is being used, if a PSC in which a vCenter Server is connected to fails, access to this vCenter Server through a different vCenter Server's vSphere Web Client is not possible. This is due to a user's SAML token from the vSphere Web Client being unable to be passed to the failed PSC, thus to vCenter Server. Unless the PSC is brought back online or vCenter Server is repointed to a different PSC in the same domain, users cannot access it.