If you Reset Invoice to Draft and then Cancel, you can change the purchase order line.  

It is also possible to change Analytic Account on the Vendor bill, if you reset it to draft.

Hi,

The `product_id` field becomes read-only when the state is set to 'purchase'. Here's the condition that makes it read-only:

'readonly': [('state', 'in', ('purchase', 'to approve','done', 'cancel'))],

If you need to edit the `product_id` field after the state has been changed to 'purchase', you can achieve this by replacing the field with a new XML tag that has the same field name. This XML tag should not have the 'readonly' attribute set to restrict editing. This will allow you to modify the `product_id` field even when the state is 'purchase'.

<record id="purchase_order_form" model="ir.ui.view">

        <field name="name">purchase.order.inherit.your.module</field>

        <field name="model">purchase.order</field>

        <field name="inherit_id" ref="purchase.purchase_order_form"/>

        <field name="arch" type="xml">

            <field name="product_id" position="replace">

                <field name="product_id"

                       attrs="{'required': [('display_type', '=', False)]}"

                       context="{'partner_id':parent.partner_id, 'quantity':product_qty,'uom':product_uom, 'company_id': parent.company_id}"

                       force_save="1"

                       domain="[('purchase_ok', '=', True), '|', ('company_id', '=', False), ('company_id', '=', parent.company_id)]"/>

            </field>

        </field>

    </record>

Removing the 'readonly' attribute to allow editing of the `product_id` field after the state has changed to 'purchase' might introduce unintended or unwanted behavior. Carefully consider the implications of allowing edits in this state to ensure that it aligns with your business logic and requirements.

Hope it helps