There are a lot of reasons why a customer is changing his hardware. A few of them are:
What ever the reasons for a replacement may be, you will face one question. What to do with the old firewall?
Licenses
The answer to this question mainly depends on the conditions under wich the new firewall has been bought.
If the customer simply bought a new device there is nothing special really. The only thing you should make sure before selling the old devices is that there is no EULA prohibiting resale.
A Trade-In means that you get a discount if you prove that you are replacing a device of a competitor. You will have to provide the serial number and manufacurer of the device that you replace when activating the firewall to get the feature key. This also means that you are not allowed to keep using or sell the old hardware.
A Trade-Up means that a device or cluster of the same manufacturer gets replaced by a more powerful / newer model. In this case you have to return the old devices back to the manufacturer.
RMA stands for Return to Manufacturer Authorized and means to replace a faulty hardware unit. The broken device has to be returned to the manufacturer.
Cheers, Ori