Unable to adjust the price for Service Activity

[shahyan1]

Once a purchase order is created from a Service Activity the amount on the Purchase Requisition can't be changed. If I would like to increase the amount, I can add further activity (i don’t know if there is any other way). But if I have to reduce the amount then it wouldn't allow.

When a service activity is created and a purchase order is created. The amount I put in the activity shows up as a Plan cost in the 435 report in version 0. I need to use the Plan cost to represent the project estimate. In order to update the Plan cost I should be able to adjust numbers put in Activities. How do I adjust the value in a Service Activity?
Thanks,