Members charged full amount if Membership is switched in the middle of a cycle

Customer has paid $10 on the 1st day, on the 7th day he wants to change membership to plan B which has amount $20, so how much customer needs to pay on the 7th day to change the membership?
Currently The member will be charged with $20 and the membership to Plan B will be counted from the day he switched. There will be no discount or reduction with the earlier payment or with days passed for the earlier membership.
so do you think this is the right scenario of membership? customer have to pay 2 full membership amount for almost same month.
An option is to use coupon to allow discount to the client but coupon is not the right option, I've total 180 membership and it is quite frequent that they will require to change the membership in my scenario, so the flow which will never gonna work with our customers.. in fact for no one it is the right flow.