I don't think it's confirmed that 10.1 will be a payed upgrade. But even if it is, JRiver has been pretty good aboutoffering price breaks on upgrades for pre existing users who switchover early.
I'm fortunate in that I have the $10-$40ish dollars to pay for the upgrade without really having to think about it. Not everyone finds themself in the same situation. That said, my only real thought on the matter is that requiring a payment for an upgrade after only 10 weeks of development -- even if that represents 4000 work hours -- might not be the best idea from a business perspective. First, you're gonna make people angry. Ultimately, though, that's not that big a deal. If it's a good product people will complain, but they will still pay.
Second, and a lot more important, is that the people using v10 have played a very important role in its rapid development. They essentially paid money to beta test. Without all those testers, bugs couldnt have been squashed as quickly and progress would have been slower. If you establish a precedent of asking for money few months, people will hold out on you and only pay every second or third time. They wont have all the new features, but they'll have something that works. On the flip side, JRiver wont have as many people to help test its product.