There are 2 "problems" with your inheritance:

1. You replaces a "page" tag with a "field" tag.  Generally it is not a good idea.
2. If you objective is to change the order_line's on_change behaviour, it is not a good idea to "replace" the field with an incomplete definition.  A lot of things can go poof, including the "tree" element that is required by the same (in this case) or other modules in other views inherited from the same parent view (view_order_form).

A better way is just to change the attribute like so (in fact the erroneous view_order_form_editable_list view is a good example for this):

<xpath expr="//field[@name='order_line']" position="attributes">
          <attribute name="on_change">onchange_line(order_line)</attribute>
</xpath>

Hi, try this one. Hope it helps. <xpath expr="//field[@name='order_line']" position="replace">