This is my sample...

In your .py inherit the class you want to change the user id fro example

class class_name(osv.osv):
_inherit = "class.name"


_columns = {
    'user_id': fields.many2one('hr.employee, 'Assigned to', track_visibility='onchange'),
}
class_name()

and in you .xml

  <record id="name_you_want" model="ir.ui.view">
        <field name="name">name.you.want</field>
        <field name="model">table.you.inherited</field>
        <field name="type">form</field>

        <field name="inherit_id" ref="table_you_inherited.name_of_the_form_where_you_want_to_change"/>
          <field name="arch" type="xml">

               <xpath expr ="//field[@name='user_id']" position="replace">
                    <field name="user_id" />
              </xpath> 
            </field>
    </record>

I hope it will work :D