Change delivery priority in sales order via user exit

Dear SD Expert,

          I have a requirement to change the delivery priority (VBAP-LPRIO) based on customer group 3 (VBAK-KVGR3) in header. I am using user exit MV45AFZZ , subroutine USEREXIT_SAVE_DOCUMENT_PREPARE, but it did not change after I click on save button.

          Do anyone have the similar requirement, kindly share your opinion or work around solution. Appreciate it.

Thanks

Wong Hock Lin