What is the likely sequence of events in performing an upgrade to a product in this
They will likely make a new publication and revoke the old one.
Will they use GPO to uninstall the previous version of the product first and then deploy
the new MSI?
This is TOTALLY irrelevant for you. MSI that are properly coded WILL trigger at least the uninstall of a previous version.
Or will they expect the new MSI to automatically uninstall the previous version?
They will assume the MSI behaves like that because this is the behavior it should have.
Edit: Tiggering the uninstall of an old version is trivial in a new version:
<Property Id="PREVIOUSVERSIONSINSTALLED" Secure="yes" />
IncludeMinimum="yes" IncludeMaximum="no" />
So, the new build CAN triger the nuisntall without additional "coding". This is purely configuration.