I can give you a zero chance of either the UBB.classic or UBB.threads code being placed under an open source license. Signifigant elements of both are being used in the new product, in fact. You probably won't recognize the code when you see it, but it'll be there nevertheless. It would be foolish of us NOT to reuse at least some of the ~175,000 of lines of combined code.
I want to just clarify a misconception - we have no plans to drop support for UBB.threads or UBB.classic in the short term. While we are no longer putting any time into adding new features to either product, we are still taking bug reports, and will be making new bugfix releases.
We will also be providing support for both products, both on the forums at UBBCentral and via support tickets.
There will come a day when we'll announce that we intend to change how we handle support of the retired products, but that's still quite a way off. By that time, it's our hope that there will be very few qualms about switching over to the new product.
As for users with multiple licenses - if you currently have both a Classic and a Threads license, then you'll simply have two licenses, and can set up two boards of any of the three downloadable flavors.
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