I did the same thing, on a sub upgrade.
Ends up Apple charges the full amount of the new subscription, and then refunds the remaining of the old one.... which results in a several day delay of your credit card having the correct balance. I find that misleading from Apple, they should show on screen that its going to do that in two transactions... or just charge the subscription delta which would be more user friendly. Letting Apple run all this crap without any checks and bounds and competition results in this stupidness and a lack of a need to improve the App Store.