Calculation :
upgrade period (months) =from last bill date for service name of extra page(last monthly statement issue date) to next bill date for service name of basic(next plan period start date) - 1 day
upgrade fee = (New monthly charge - old monthly charge ) x upgrade period
* Clients need to pay all extra fee before upgrade(If clients own)

e.g a/c : 2300XXXX
old plan : $29/m , F150, prepaid 24 months
new plan : $39/m , F400, prepaid 24 months

In the billing info for this a/c,
service name monthly charge last bill date next bill date billing period
basic 29 2007/6/26 2009/6/26 24
extra_page 0 2009/4/26 2009/5/26 1

upgrade period = 2009/4/26 to 2009/6/25 = 2 months
upgrade fee = (39-29) x 2 = $20
New billing will send on 2009/5/26, the bill will show the balance of last period $20 and add extra fee (If clients own)
The extra page fee before 2009/4/26 must be pay, cannot be waived.

How much of extra fee ?
Please check the amount with invoice date on 2009/4/26 or latest invoice for this a/c.

upgrade plan start date: If clients don't have assign date, we will assign last bill date for extra page is a start date