I suggest you sign out of the Adobe application manager and sign back in to see if that resolves the issue.
Just get the serial number from the other PC under Help>System Info
As far as Lightroom and Adobe knows you have installed LR on to many computers. A new motherboard with new CPU is a New Computer. You can try signing out and then back in but that may not work as you didn't sign out when the older MB & CPU was installed. In that case you will need to contact Adobe directly.
Thanks for the suggestion. It is now coming up with a message at the start up that there are now too many installations!
As the 'installation' is actually on the same drive as before - all that changed was the motherboard and processor - I assume it sees this as a new computer/installation.
Looks like a chat to customer support.
I appreciate your help so far. It was a good try.
It doesn't matter about the same drive. It is on a Different Computer.
Hopefully a STAFF member will see this and straighten it out for you.
Adobe should really include a Web Based system to Deactivate installs. Like Microsoft does with their Office 365 subscriptions. Just sign in to your account and you can Deactivate any and all computers you have Office 365 installed on.