A software registration question

I can't figure out where else to ask this question!

My computer is pre-loaded with a sixty-day trial of the latest Office
software. We already own the last version so what we really need is an
upgrade. Problem: My husband can't find the disks! We were going to install
those and then upgrade on line.

I have the product key. Does anyone know if I can install the trial-period
software and then use my product key to identify myself as someone who
already owns office and just needs to pay to upgrade?