https://userapps.support.sap.com/sap/support/knowledge/en/2440191
2440191 - Changeability of BKPF-XMWST in FB02 and FBD2 | SAP Knowledge Base Article
There is a requirement to change 'Calculate tax' check box (BKPF-XMWST) for an already posted document in transaction FB02 or FBD2.