In my lab environment I was running Windows vCenter 6.7 and with the release of vCenter 7 a migration is needed because there is no Windows vCenter anymore.
The following articles will give you enough information on how the process works especially the how-to from Vladan Seget:
https://docs.vmware.com/en/VMware-vSphere/6.5/com.vmware.vsphere.upgrade.doc/GUID-9A117817-B78D-4BBE-A957-982C734F7C5F.html
https://www.starwindsoftware.com/blog/how-to-migrate-vmware-vcenter-from-windows-to-vcsa-6-7-update-1
Basically the process is the same for vCenter 7 with in my case one issue.
At first try my migration failed at the stage when the migration assistant is shutting down the Windows vCenter and the VCSA 7 is being brought up with the original IP, hence resulting in a conflict and the upgrade is in a broken state. I followed the process and saw that especially Windows Server 2016 has the annoyance to delay the shutdown for minutes, this is a known issue and happens from time to time and to my knowledge has to do when updates were installed (even after multiple reboots).
So the migration assistant is doing its job and completes the migration and is shutting down the VM, except in my case. Resolution for this is when the script is completed and windows says it’s still shutting down etc. do a hard power off and after that the migration is moving further as expected.
In summary:
1. Make sure Windows is up to date with all the updates and there are none outstanding
2. Make sure vCenter is up to date, in my case it was the latest update of 6.7
3. Make a snapshot before you start ? saves a lot of hassle later on
4. Force kill the Windows vCenter when it’s shutting down after the migration assistant completes
After this the migration in my case completed flawlessly.
Hope it helps!