Should you choose to change your plan halfway (or midway through your billing cycle), Narrative has things sorted so you don't end up spending extra.
Narrative will give you a partial refund for the period of the month (or year) you did not use your old plan and the partial amount for the new plan will be placed on your next bill.
If you are billed $10 on the 1st of each month and you change your plan exactly halfway through the plan’s billing cycle (i.e., on the 15th of April). In that case, you have already paid $10 and used half of the current billing cycle. When you switch to the new plan, with a cost of $20, the unused portion of the previous plan results in a credit of $5. The cost of the new plan for the rest of the billing cycle (i.e., the rest of April) is $10. Therefore, the total prorated cost of switching from the cheaper plan to the more expensive plan is $5. This amount is added to the next invoice.
The next invoice, on May 1st, also reflects the payment required on the new plan for that upcoming month: $20. Thus, the May 1st invoice is for a total of $25.
In this example, you are not billed the $25 at the time the plans are switched. The plan change impacts the next invoice, which remains on the same billing cycle (as both plans have the same billing interval).