Since you have an MSDN license, you are technically required to do a clean install if you want to use your MSDN key.
If you own any valid upgrade key (even if you've already activated another installation with it), there is a trick to make anytime upgrade work. Run anytime upgrade using the regular upgrade key, and uncheck the box when it asks if you want to activate windows automatically. Then, once the upgrade is finished, go to the System control panel and click on "change product key". Enter your MSDN product key and activate with that. I've done this before (with MSDNAA, but it should work the same) and it went off without a hitch.