Changing Subscription

Hi,

I have a question about what happens when I edit/change a subscription.

When a subscription is created, it automatically pro-rates the amount from the date it was created, until the next billing date - this is good.
When I edit a subscription and change, for example, the plan, it prorates from today until the next billing date. However, the issue arises with what happns to the "old" plan. When the "new" plan is less per month than the "old" plan, Zoho doesn't seem to properly credit the account in this case, so the customer is billed extra.
Also, if the quantity were changed from '1', to '3', Zoho automatically creats a new bill for all 3 items, rather than only the 2 "new" ones that I just added to the existing 1.
Is there a way to have it automatically credit the account in these cases?

Thanks,
Edward