Hi,
If Modify Header is executed without modifying anything, some data of the lines could be modified because of the on_change_product.
For example, if you add some tax manually different from the taxes that would fill the compute_taxes, it will be removed or changed after running the wizard.
I think that if nothing is changed at the wizard, nothing should be modified.
Why not but the same apply to purchase, you can follow Tryton - Get Involved