Without having to get a support tech to do it for you. What would be reasonable to me is if Adobe didn't just show you your product serial number when you look at your products, but allow you to simply wipe clean your registration status and start over. Problem was, now I couldn't sign back into the old PC's! As of last night, (when I finally had my first bird pics from my new 500pf lens to play with), I suddenly had ZERO working Lightroom systems. I then signed out of my spare PC, but again no joy on the new PC. I signed out of my laptop, but I couldn't get signed in on new PC. To the unitiated, unregistering Lightroom is indeed pretty much signing out of a web site, though you do it from the Lightroom Help menu. The adobe server sees two PC's, I need to unregister one.

Never had a problem until trying to get it working on the latest PC build where I got an error that the serial number was already in use on the maximum computers allowed ad that I would have to sign out of one of them in order to register the newest PC. I also have a spare PC that might get used somewhere. One is my PC, the other is usually a laptop on a trip. You are supposed to be able to use it on two computers. Over time I've migrated it from one older PC after another, passing through three PC's and two laptops. I registered Lightroom in 2015 and have upgraded it to 6.14. I had to call or chat with Adobe support on this one, though I'm not sure why this happened at all.

Adobe lightroom 6 serial number