Isn't this just breach of contract, plain and simple? Change of ownership should have little to do with it.
The only recourse is taking them to court which will end up costing your time, money, and sanity.
Time used so far: 4 years
Time remaining with new license: 2 years
Time lost due to license change: 20 years
Damages awarded to the plaintiff: $30
Plus the opportunity cost of your lost time.
https://web.archive.org/web/20190508161003/https://manycam.c...
But the wording allows for many options.
A lifetime license, to a specific version. With no updates. A lifetime license for upgrades.
https://forum.manycam.com/uploads/default/optimized/2X/0/0da...