SCCM 2012 R2 Primary Site to join SCCM 2012 SP1 CAS

I was doing some experiment yesterday to try out the scenario above. I’ve setup a SCCM 2012 SP1 CAS, then install a SCCM 2012 R2 primary site and try to join to the CAS 2012 SP1 hierarchy. Guess what…… It failed…… The setup wouldn't allow the setup to continue because it detected the new installation version is newer than the version installed on the CAS.