Windows Server 2000 TS CAL
Windows Server 2003 TS CAL
Windows Server 2008 TS CAL (2)
This may be for lack of understanding RDS, but why in the world would 2012 R2 allow you to migrate/install server 2008 RDS CALs if those CALs can't be used by end-points to connect to a 2012 RDS Session Host?
To the previous comment: the rdsh server can be 2008 r2 that uses a license server that is 2012 r2 with downlevel calls installed on it.