There might have been some misunderstandings / misinterpretations of your initial supporter.
Simply joining a machine never required so far any schema change.
The schema defines what can be stored how in AD and has not too much to do with the domain join process.
Having said that, depending on the functional level / DC OS level certain new security features like encryption mechanism have been introduced.
If you simply join the 2012 server you don't need the latest schema.
But certain features you may want to use require new schema or adding a 2012 DC or even updating the functional level. Claims based access control would require new schema and a 2012 DC