**EDIT** If you log into vcenter with an Active Directory account you should be able to modify an already existing Identity Source. I had been logging in with local administrator account.
For reference we already had our linked vCenter talking to Active Directory over LDAPS. However, we are currently in the process of migrating all of our VMs over to new hardware. When we tried to move the main Active Directory server providing authentication to vCenter, lets just say it was not happy.
Upon trying to enter into the Identity Sources and update the server(s) manually on the Identity Source that was already being used we received the following message: “Check the network settings and make sure you have network access to the identity source”.
It was not found until after doing some Googling that you have to remove your current running Identity Source in order to make changes. In other words delete the current identity source and add a “new” one in order to make the changes you want to make.
This just seems bad.
However, after doing a lot of testing in our TEST environment I could not seem to run into any snags. If you login with email@example.com and delete and then immediately re-add the identity source back with the same domain name, alias, etc, there does not seem to be any issues. All of your permissions on objects defined with AD groups will remain.
I used the method listed in this VMware KB for grabbing the certificates I needed for both the Primary and Secondary Active Directory Servers. (https://kb.vmware.com/s/article/2041378).