Generally, after MS releases a new server OS, you can only buy licenses for the new OS, but you have downgrade rights to the older version(s). I'm pretty sure CALs work the same way. If that's the case, you can buy a 2012 RDS CAL and use it as a 2008 R2 RDS CAL.
I don't work for MS and I'm not a lawyer - just an IT guy that has suffered thru plenty of license hell before.
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