Diagnosis
You wanted to change a planning entry which is not an original one butan entry which was implicitly generated as a periodic followup. You mayonly edit the first entry with a start time in the future from such afollowup row.

System Response
The requested change was not applied.

Procedure
Look at the period of the selected planning entry and go back to olderentries in this period steps until you reach the oldest such entrywhich has no scheduled start time in the past. This is the anchor entrywhich you may select and apply changes. The change will affect also thefollowup entries.