there is a field called "priority" on model "ir.ui.view" and default it is set to 16. if you made a complete new form view for purchase.order, just set this field to a lower value. the actions on the purchases menus for purchase.order aren't set to any special view, so the view with the lowest priority will be chosen automatically.
edit: if you want to have a menu wichh opens a form view of purchase order with your defined form view, you have to use it in an action, like:
<record model="ir.ui.view" id="your_form_view_id">
... your form view ...
</record>
<record id="your_action_id" model="ir.actions.act_window">
<field name="name">Menu Name</field>
<field name="type">ir.actions.act_window</field>
<field name="res_model">purchase.order</field>
<field name="view_type">form</field>
<field name="view_mode">tree,form</field>
<field name="view_id" ref="your_form_view_id" />
</record>
<menuitem action="your_action_id" id="your_menu_id" parent="your_parent_menu_id" />