Thanks. I'm aware of the best practice of moving to ms-ds-consistencyguid, but currently I am looking at migrating to a new Azure AD Connect instance like-for-like.
If the existing Azure AD Connect instance uses ObjectGuid as the source anchor attribute and I select ms-ds-consistencyguid in the new Azure AD Connect installation wizard, then the matching will break and Azure AD Connect will do some bad stuff - likely try to create new duplicate objects.
To clarify my questions, I was more interested as to why the Azure AD Connect installation wizard was trying to automatically select ms-ds-consistencyguid if the current instance is ObjectGuid, and wanted to be sure there wasn't something awry going on.
I have performed some further research since and I think I understand why it's trying to guide me to use ms-ds-consistencyguid. I installed a new Azure AD Connect instance in staging mode and selected another source anchor attribute. When I try to install Azure AD Connect from a different server, it now wanted to autoselect that new source anchor attribute. This is a flaw in the product since it means the value in Azure AD does not necessarily match the attribute used by the production AAD Connect instance.