Moderator: Moved to vSphere Upgrade & Install
The first step is to upgrade the external PSCs. Then you can upgrade the vCenter servers. I don't think you need to upgrade the VUM servers as VUM will now be part of the VCSA.
After the migration you can use the vCenter Server Converge Tool to move from the external PSCs to embedded deployments.
BTW if you have some sort of cluster right now, you can move to vCenter HA.