Prorate plan for current period only

We have a plan $40 per year. We are adding a new plan (upgrade) for $60 per year.

If someone tries to upgrade today, it seems to renew AND extends their subscription to today. It does prorate the amount but actually seems like more than the difference between the plans because it extends the date.

I want to just upgrade their subscription for the current (same) period.

Plan A 4/3/15 to 4/3/16 user paid $40
Plan B upgrade today, $60 annual. They should only have to pay $10 to upgrade through 4/3/16. (6 months of the difference) System actually wants to charge $30 and extend expiration to 8/3/16. We don't want that.

Is there a way to it keep the same expiration date?

Best I can suggest is somehow setup a coupon for upgrades to that plan to discount it further, but aside from that no it's not possible as the renewals do not function that way. It basically just upgrades to the new plan and discounts it based off the prorated amount. The duration of the upgraded plan is still used in all of this calculation as it's an upgrade and not a replacement.