You can no longer change the system for planning and SA release creation

What causes this issue?

You wish to switch the system for planning and SA release creation (
<LS>Ext. Planning</>).
This is not possible because delivery schedule lines already exist for
this scheduling agreement.

System Response

You can only switch between planning and SA release creation in the
standard and APO systems as long as no schedule lines have yet been
created.

How to fix this error?

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.