As I understood you want to change the domain.
Here is step
1. Take down time for VC and PSC
2. Leave the VC domain some time its done by command line
3. Do the same thing on PSC
4. Reboot VC and PSC
5.Put new DNS IP
6. Join new domain VC and PSC and reboot
If its successfully done and you are not able to logon VC then you need to power off VC first then PSC
2. Power on PSC then VC.
The option to join vCenter Server Appliance 6.x to an Active Directory domain is unavailable in the ...
IF you got any error let me know
Rajeev Chauhan
VCIX-DCV6.5/VSAN/VXRAIL
Please mark help full or correct if my answer is use full for you