You are unable to edit/change the Customer details in Project with In planning status
Reproducing the Issue
Go to Project Management work center
Go to Project view
Search for the Project XYZ
Choose Edit button and Project plan
Now navigate to Accounting section
Here you can see the Customer details are not editable
If the project is created from sales order the customer details are not editable.
If the project is associated with a sales order the customer details are not editable.
If the project is not associated with a sales order, only then the customer details are editable. System works as deisgned.
Project sales order customer , KBA , AP-PRO , Project Processing , AP-SLO-PRO , Sales order project processing , How To