How do I deactivate Ghost 15.01 on an old system so I can activate same serial # on new system?

We're building 5 new systems and donating the oldest 5 systems in our computer learning center.  I want to deactivate Ghost 15.01 on the old systems so I can use the same serial numbers on the new systems we're building.  I've searched Help on the product with no success and I've also searched this forum with no success.  It must be published somewhere, what am I missing?