Where do i find my Adobe serial number?

Hi All

I work on an IT Helpdesk and have a user who has upgraded from ADOBE Acrobat Standard DC to ADOBE Acrobat Pro DC, however he isn't able to complete the installation because it is prompting him for a serial number of the previous Acrobat Standard DC. That App has now been upgraded so he cant access it to locate a serial number, and he cant access Pro DC because he isn't able to log onto the previous version in order to locate the Serial Number.

So we are stuck, purely because we do not know the previous serial number, do not have the original CD, and despite looking in the registry I cannot locate the key.

Where should I be looking in order to obtain the Serial Number for a product that no longer exists, in theory, but is in reality an upgraded product, if that makes sense!!

Any help greatly appreciated