I had a CS6 trial that expired. After seeing the Creative Cloud offer (for $30/mo) I decided to sign up for it.
I signed up, but my Photoshop CS6 would still say the “Your Trial has Expired” and ask for me to “License This Software”.
It was licensed!
After clicking “License This Software” I’d log in with my Adobe Creative Cloud account, and be asked for a serial number. Despite many internet searches I couldn’t find a way to get a serial number (you can’t, Creative Cloud doesn’t work that way).