buy new license and transfer old license

I am a university researcher/prof using FF in a chain of tools with my grad students. I own a few copies of FF4 for different computers. I have a new student that I need to give a new copy too. So with FF5 out, I thought I buy new FF5 for my computer (current w FF4) and move my current FF4 license off my computer and give that to the new student for their computer. I am in the midst of important work with many scripts on my computer so just want to make sure I do this correctly. Can I just use the newly bought FF5 license key to replace my current FF4 key and then give that FF4 key to my new students. Ort do I have to install ( and if so will that kill my scripts and library setups). Again I am trying not to lose my current routines and setup which at this point it pretty complex.

Suggestions for best way to handle this?
thanks in advance

I was not clear - I want to move up to FF5 on my computer (by buying a new FF5) and give my old FF4 license currently on my computer to my student, in a way where it is least disruptive to my computer's FF setup of scripts and filters.

thanks