Remove 2012 domain controllers from a 2008R2 domain due to licencing

Hi

We have recently taken over support for an organisation and discovered that even though they only have CALS for server 2008 a number of 2012 domain controllers have been installed although the functional levels have not been raised, given the number of users involved upgrading the CALS is not going to be an option, we are considering demoting and removing all of the 2012 domain controllers and replacing with Server 2008 DC's, is this possible or would removing them cause issues resulting from any extensions / modifications of the schema that may have happened when they were installed.

Thanks

Nick