I mentioned this a few months back when they announced they were doing the migration…I believe Bubble has it sorted in that, we are migrated on our renewal date as you’ve indicated in this thread…that means, nobody would be charged for previous WUs, because the situation in which a user is charged for WUs from earlier in their billing period is when they migrated before the end of the current period.
For example, your current period billing is August 12 - September 11 and you migrated on September 1, Bubble would charge for the WUs from August 12 - September 1…so for a user in this situation, I believe Bubbles approach is to migrate automatically on September 11, which should result in no charges fro WUs from earlier in the period.
My money is they understand the costs of having to fix the issue in retrospect with lots of dissatisfied customers is greater than ensuring pre-emptively that there will be no cases like this from their planned migration.