With your concern, when you upgrade from PRO to UNLIMITED, the charge is prorated based on the difference in price and the number of days remaining in the billing cycle according to Shopify Policy.
- You begin a 30-day billing cycle on a PRO plan, which is $9.00, and then upgrade to an UNLIMITED plan, which is $14.90 plan on day 10 of the billing cycle.
=> You'd be charged ($14.90 - $9.00) * ((30-10)/30) = $3.93.
- Simply, you upgrade from PRO to UNLIMITED on the same day, you'll be charged for the difference $5.90 only.
I hope this will be clear your concern. Let me know if there's anything else I can help you with.