Prorated By Price Not Working for Account Upgrades

I am currently testing my membership site. I have two plans:
Monthly Pricing Plan - $39.99/month
Annual Pricing Plan - $409.99/year.
If a member would like to upgrade their account from monthly to annual subscription, I would like them to be able to take advantage of the prorated pricing. I set everything up the best of my ability in the "Subscription Plan" settings for the "Monthly Pricing" membership plan.
In the users profile, their is a link that allows them to upgrade from monthly to annual. The link shows the prorated price but when the user clicks on the link, they are taken to the "Upgrade Subscription" form. This form shows the regular pricing ($409.99), not the prorated price. The regular price also shows up as what they will be billed when they pay with Paypal standard.
Note: Paypal Standard is the only gateway I am using for this site.

Hmm

At the moment, the system only support upgrade from one time subscription plan to another one time subscription plan.

It is not possible to upgrade from recurring to recurring plan yet (as when upgrade, we need to cancel the original subscription plan, but most of Payment Gateways don't have API to allow that)

So maybe the error happens because your plans are recurring plans? Could you please try to upgrade from one time plan to another one time plan to see whether it works?

Tuan