Diagnosis
Changing the start date of the price plan leads to a different budgettype.

System Response
The system refuses this action while there are still materials in theprice plan.

Procedure
If you want to keep the materials in the price plan, copy the price planto the new date. The copy process has a transfer function for materialsin the case of a change of budget type, which transfers all materialsthat can be copied consistently to the new price plan.
Otherwise, delete the materials from the price plan and change the startdate.