When an upgrade or a downgrade is made to your plan in the middle of a billing cycle, pro-rated charges or credits will appear on your invoice.
For mid-cycle plan changes your Sprint.ly invoice will show the following:
Prorate the remaining time on the existing plan and issue a credit
Prorate the new plan until the current billing cycle’s end date and invoice that amount
Invoice the outstanding balance (charges minus credits)
If the charge is greater than the credit you will will be invoice the difference. This is the likely scenario for plan upgrades.
If the credit is greater than the charge, then you will have a credit on your account that will be applied towards your next month's subscription fees. This is likely the scenario for plan downgrades.
All plan upgrades or downgrades are immediate where as plan cancellations will take effect at the end of your billing cycle.