This is what Bubble support has replied to me.
When you switch from a Legacy plan to a new Workload-based plan, you will be charged for all previous Workload Unit (WU) usage for the current calendar month. This means that even after downgrading, any workload that was used while on the legacy plan during the current month will be billed.
In this case, I would recommend waiting until October 1st to switch to a new pricing plan. Workload units are calculated based on the calendar month, not your billing cycle, so that would ensure you are not charged for any workload units accrued in the month of September.
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