Can we make Planned Effort fixed when entering ETC Quantity in PJT?

When updating progress on a task in PJT, standard functionality is when changing the ETC quantity the Planned Effort is automatically calculated on the form. Example: Original Planned Effort = 73, Actual Effort = 53, And ETC shows 20. We decide that ETC is actually 30 and update ETC, which in turn changes Planned Effort = 83.

Our PM's would like to be able change ETC without effecting Planned Effort. Is there a way to do that or id the only course to create new forecasts and look at Orig vs. updated forecast?