When adding an operation or changing an operation on an active production order in Production Planning, is there a way to see the new estimated hours in the Estimated vs. Actual Hours costs session?

When the WO(production order, also called work order) is in active status, you can't compare the Estimate and Actual hours. Because, at this moment, the Actual hours has not been reported, havn't been inputed for the WO.

The actual hours data has been key in in BP(people management) with (bptmm1111)session. or you can use the backflush function to report the hours automaticly. only after this action, the actual hours is available.

Not sure what version you are running, but it may be worth looking in Shop Floor Control Parameters (Baan IV and earlier) since it may be a parameter setting that is stopping the automatic update of the estimated hours.