Hello.
You may have had a bill generate on the account before the upgrade took place. For example if your bill generated on the 4th of May, payment would have been due some time between the 14th and the 23rd. The next bill would be generated on the 4th of June, and would be billed at the plan rate.
You can check the bill date within My3. My3 should also show you the details of your new plan so you can verify that it's the right one. If it isn't, I'd recommend reaching out to the support team for that to be looked into.
Pete.
