As usual, your bill will include the cost of your plan for the next month.
As each month is paid in advance, you’ll get money back for any unused days of your old plan from the previous month.
This covers the days between upgrading and your bill being produced (the same days that you got a refund for on your old plan).
If you’ve added any charges to your bill, for example the upfront cost of a device or a delivery fee – this will show as a 'one time charge' on your bill.
If you exceed your monthly allowance of minutes, texts or data this will also be in your first bill.