as far as i know you buy a license independent from the hardware...
meaning that you can MOVE the license to any server you want as long as it is just 1 server using that 1 license....
so its save to remove that old server, and install avast with that license on the new server
suppose you have a 3 year deduction period on your hardware until u buy new hardware
then u buy in the last year of that period a 3 year license for avast server then u replace the hardware but still have at least 2 years of avast left....
seems to me that you can use that 2 year on the new hardware until its time to extend the license for avast