It depends on what kind of subscriber you are. If you bought a new Family or Premium plan, or restarted an expired one, on or after October 8, 2026, you already have the shared 2TB.
If you are an existing subscriber, you keep your current storage until your first renewal on or after May 2, 2027. That means the switch will not happen for everyone on one day. If your plan renews in August 2027, for example, the change arrives then. Microsoft says it will email you before your renewal.
Two details can catch people out. Upgrading from Family to Premium before that date moves you to the shared pool straight away. And at least one Microsoft page lists April 1, 2027, as the reference date instead, so treat the email Microsoft sends you as the final word on your own timing.
You can check your renewal date on the Subscriptions page of your Microsoft account, and see how much space your family uses under Microsoft Storage.