When importing a CRM 2011 organization into CRM 2013 UR2 or SP1 environment, you may receive a cryptic message “Database having version 126.96.36.1999 is not supported for upgraded”. As it turns out, it has nothing to do with the version of the database being imported.
If you are trying to import an organization database that has already been imported to an organization on that CRM server, you will get this error, because the organization has the same ID.
Deleting the other organization will do it. If you need both organizations, you can delete and re-import the already upgraded organization. This will assign it a new organization ID. Then you can proceed with upgrading the second copy.
(The resolution was provided by my fellow tipster Joel but, in a good “first come first served” tradition, I’m staking the claim.)