I do not know of a way of changing (and keeping) changes to screen fields, without modifying xml files.

It is possible to change the visibility of a field on the screen, you have to modify or create an XML-file. The advantage is also that this way you can reload the database without loosing your changes.

When a field is invisibile, it can store / keep values and/or the values will be stored in the database when saving the data. Sometimes a field is defined multiple times on a screen, so you hide one, and the other is still visible.

Example of hiding a field (in this case, hiding the taxes on sale.order.line):

  <xpath expr="//field[@name='order_line']/tree//field[@name='tax_id']" position="replace">
      <field name="tax_id" invisible="1"/>
  </xpath>

It is also possible to use attr{} so you can make it invisible depending on some things.

  <xpath expr="//field[@name='order_line']/tree//field[@name='tax_id']" position="replace">
      <field name="tax_id" attrs={'invisible':[('field name or property', '=', 'some vaue')]}/>
  </xpath>

Hope this helps you a little.

try it.  Sure!

            <field name="arch" type="xml">    
                <xpath expr="//field[@name='order_line']/tree//field[@name='price_unit']" position="inside">
                    <field name="serial_no"/>
                </xpath>
            </field>