Hi,
It would be good if you can run a query to correct the siteguids, but I don't really think that is the cause of the problem.
However what you posted about SiteGuids I guess you mean they all have empty guids ie: "00000000-0000-0000-0000-000000000000" then you need to update them to new guids using
UPDATE mp_Sites SET SiteGuid = SELECT newid()
But then other data should ideally also be wired up with the correct siteguids in other tables. Some tables also have SiteID which could be used in queries to update them to the correct siteguid.
But the above may not be the problem about access denied.
You are upgrading from a very old version and I think we did change something at one point a long while back about how roles are stored.
I would look in mp_Modules.AuthorizedEditRoles and mp__Pages.EditRoles
If it looks like the affected rows have "Admins" as the value, run a query to update those to empty string and see if that solves the problem.
Hope that helps,
Joe