You would tell them what you did, they should return your licenses. Also, you may be able to reactivate the keys, since they are multiple activation keys, depending on the number of times they've been used, etc.
See this FAQ:
https://technet.microsoft.com/en-us/library/ff678211#faq3
Specifically:
"How do customers get a MAK activation limit revised?
Customers can check their MAK allocation limit, check remaining activations on the keys, and request to increase the activation limit by contacting the
Volume Licensing Service Center (http://go.microsoft.com/fwlink/p/?LinkId=184280).
What do I do if I use up all of my MAK activations?
Contact Microsoft and explain the situation. You will be provided additional activations (under reasonable circumstances)."