I set up a Virtual Machine and simply reinstalled QB 2021 on that machine.

Make sure that you don't enable internet access to that machine as that is what triggers activations etc. ie. set system date manually.

Simply copy the data files into your directory along with the Attach folder which holds your scans, re link them,  and hey presto, of she runs.

I think the way Intuit changed the model so that people like us who had bought the product since 1996 ended up on this subscription thing which allowed intuit to bully us into the online version was scurrilous. And to not even let you access old data, well, what can you say..............