Can upgrade license keys be reclaimed?

Hello, I have a question that I need to have answered:

We have a couple of DOMs that we use as backups in case we need to replace our customers' units. Also, it's easier for us to upgrade our existing customers if we just swap out their DOM for one of ours. The problem is that some of our customers have a 4TB DOM and others just have a monolithic 16TB DOM. Well, one of the slush DOMs that we have now is a 16TB DOM that had been upgraded from a 4TB DOM (via a 4TB upgrade and 8TB upgrade key). I'm wondering if it's possible to remove the upgrade licenses. That gives us additional flexibility.

Unfortionatly we cannot remove the Lic Keys then use them on another DOM. The reason is that they are married to the SN of the DOMs.