I have a new key fob for a 2012 MB E-class that I bought from MB for a spare. I am looking to purchase a 2012 MB CL550. Can I get the fob reprogrammed to the CL550?
Not really, for the most part once Mercedes keys are programmed they are burned to that programming (if you ordered from MB then its pre-programmed already).
While it is possible to unlock the key and reuse it. It is rare and not many have the equipment to do this however. Mostly unlocking is only used for locksmiths who want to refurbish used oem keys for use later. Even if you found someone with the equipment it would likely costs you more to unlock and then program, then to just get a new fob all together.