Just wanted to chime in on this thread from my experience with the 21105 status. I had cleared the purchase history of my sandbox test user, and I was getting the 21105 status for about 10-20 minutes. And, then after that it was okay. My guess is that it took a bit of time for the clear purchase history action to take place, so that the user could then subscribe again. And, also guessing that if you have not cleared purchase history, then you would receive an error message.

You can create 2 different subscription groups, however, the problem I see is that if a legacy user cancels their subscription, then they will always have the option of renewing the subscription for the legacy price in the future using the settings app. The ideal approach is to only allow the legacy price if the user does not cancel their subscription, but I don't believe this is possible with the current app store connect options.