Do I have to pay the full price if I upgrade between plans?

MySudo for iOS:

No, you will be refunded a prorated amount of the original subscription price.

For example, if you purchase the SudoGo plan and then decide to upgrade to SudoMax before the end of your renewal cycle, you will immediately be upgraded and receive a refund of the prorated amount of the SudoGo subscription. Your monthly renewal date is now the day of the month you purchased your new plan. 

All purchases are calculated and handled directly through Apple.

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MySudo for Android:

Yes. You will pay the full-price for the upgrade and the current plan price will be prorated and added to the new plan's expiration date.

For example, If you purchase the SudoPro yearly plan ($50) and then decide to upgrade to SudoMax yearly days later, you will have approximately 3 months added to the 12 months of SudoMax. So your expiry and renewal would then be 15 months from the purchase date of SudoMax.

All purchases are calculated and handled directly through Google.