Buy up membership and credit unused membership fees towards new purchase

Currently there is not a way to allow a member to upgrade their membership through the system and apply any unused amount of a current billing cycle towards the purchase of an upgraded membership. A member can only change an purchase a new membership, but if that was an annual membership they've lost anything they paid towards the one they have or they have to wait until their membership expires to upgrade. 

Example: A member has purchased an annual membership for $120.00. After 5 months they decide they would like to upgrade to a higher price membership. When they change their membership and choose a higher cost membership, the system would prorate the remaining amount of their current membership and allow them to apply that as a credit towards the purchase of the new membership. So in this case, the member would have $70.00 applied to the first payment of their new membership.