Solution: As per the SAP standard behavior, mass changes to WBS element is not allowed. You can find the reasons mentioned below.

1. Changing the WBS element in a Purchase Order after it has been created can lead to various complications, particularly if GRN/SES or invoices have already been processed against that PO. It’s not a standard process.
2. As per SAP standard practice, WBS element is associated at the time of PR creation, and it gets copied to the subsequent PO. If you need to change the WBS you usually must cancel all dependent documents and recreate them with the correct WBS.
3. Also, for WBS element, the start date of the validity should usually be before any PR/PO creation, ideally when your project or operational planning starts. If the Start date of profit center /cost center used in WBS is after the PO creation date, it may lead to issues or inaccuracies while creating GRN/SES. It might not prevent creating, but it can cause inconsistency in the allocation and cost reporting. The best practice is to ensure that the WBS element, profit center and cost center are all correctly determined before a PO is processed. 

Thanks, and Best Regards,

Ganesh