I am not with Laminar Research; just a flight simmer running a disc version.
Load your original disc 1 and follow the instructions. The bad news is that there is no product key available for the disc version. From memory you will be asked for a product key or asked to insert the disc. The only way you can run X-Plane is to have disc 1 in your drive at all times.
I am in the same situation as you will be.
If your product is from Aerosoft the "registration key" they provide is a sales gimmick to entice you to purchase more of their "stuff" at a reduced rate. I got caught the same way and after an email to Laminar Research I was given the same news as I have given you.
To preserve the integrity of the disc set it has been suggested by others in the know to create an ISO disc and use this in the disc drive instead of the original disc. Consult Dr Google on how to do it. Seems simple. There is an option to create a USB dongle as an alternative.