Ok you are missing the point I’m making. You bring up a good point about what would need to be done with existing accrued paid game time, if there was a move to f2p / premium season pass, but you go right to the negative hypothetical situation where they don’t address the issue. Who is to say they they go that way and do nothing?
They could refund prepaid game time, likely a conversion to battle.net balance, as one scenario.
I don’t see why you didn’t also explore potential solutions to the problem you proposed, and instead focus on the negative reaction people would have if they did nothing to address prepaid game time.