You're only using xpath on the contents of the arch tags. The root tags should always be /form, /tree, /search, /kanban, etc.

You could also make life a lot easier by just using two slashes then just entering the field you're looking for. Unless you're on a large, complex form with more than one instance of the same field listed, it should be fine. If that's the case, just add in a few extra bare minimum details to the xpath string to find it. So for example, putting "new_field" in place of "original_field":

<xpath expr="//field[@name='original_field']" position="replace">
    <field name="new_field"/>
</xpath>