@Shiva512 

Are you able to confirm that the validation servers are still operational for pre-2021 desktop products?

I can confirm this fact. We can still support our clients to reinstall their old non subscription licenses and reactivate them.

I have no reason to believe my license would be "flagged", and that was certainly not mentioned by desktop support.

You can try to uninstall your QB program on the old PC before reactivate it on the new one. Another option, use the trial code as the first step and change it your installation code later.