Before you cancel or downgrade the subscription, you must have users save their OneDrive for Business files to another location to remain within the new OneDrive storage limit.
Any customer data that you leave behind might be deleted after 30 days, and is deleted no later than 180 days after cancellation or downgrading. As an admin, you can still access and back up data for your organization during this period.
For more, please refer to: https://learn.microsoft.com/en-us/microsoft-365/commerce/subscriptions/cancel-your-subscription?view=o365-worldwide#save-your-data
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