Update sales order item route on save

Hello gurus,

My requirement is to update every sales order item route VBAP-ROUTE on the save of the sales order :

I am using userexit USEREXIT_SAVE_DOCUMENT_PREPARE and I update table XVBAP[] (field ROUTE)

Everything is working fine but the schedule line dates are not updated !

If it's done manually :

How can I update schedule line dates programatically in the user exit USEREXIT_SAVE_DOCUMENT_PREPARE ?

PS: It's mandatory to do it on save.