I would suggest you to make a clone of vCenter, snapshot and full backup of vCenter DB as well for VUM DB backup if you are using update manager. Then you can proceed the upgrade if you want to use windows platform vCenter else you can choose migrate option to migrate the VCenter from windows platform to VCSA appliance.Nawals
I suggest writing a clean plan with all of the details, include:
1. Before any upgrade operation, please check the VMware HCL and compatibility of your existing servers with the vSphere 6.7. If you see any incompatibility, I strongly suggest do not upgrade! call the vendor and get advice for what to do.
2. In the 1st operational action, upgrade your Windows-based vCenter server to the VCSA version 6.7 u3. Don't use windows anymore, because of the future of vCenter will be only the VCSA. You can achieve this via the VCSA installation wizard (run it and you can see the 3rd option is about the W->VCSA migration). However, you should take a backup from the old vCenter server before running the upgrade operation.
3. If steps 1 & 2 are successful (without any problem) then you can start the ESXi hosts upgrade operation. It's better to use the vSphere Update Manager (VUM) for upgrading the ESXi hosts, while you can execute the ESXCLI for this goal too.Please mark my comment as the Correct Answer if this solution resolved your problem
As soon as the new vCenter pick up the existing hosts the HA would be reconfigured by vCenter. Thats the only thing which came in mind and take a look to if you role back. I think its not a problem but you should disable HA and reconfigure it if your perform the role back.
We have VCSA 6.7 and different Clusters with all kind of ESXi Hosts versions.
I manage >= 30 vCenter upgrades in the last 3 Years and its not such a biiiiiiiiig deal. Small issues and most of them have to do with 3rd. Party plugins or VUM. Related Problem prior to the update are password and cert issues.