@Pearwasp Sorry, PM is prepaid service. If you Change plan in the middle of a cycle and click "Change Now", PM will charge you the new plan amount immediately and a new cycle will start. There is no refund for the unused portion of the old cycle with the old plan.
We usually recommend people to click the option to change plan on the next billing date to avoid any loss, unless you urgently need the extra data or features on the new plan