Steps to reproduce:
- Create a product and complete a sales order.
- Then try to change the product type.
- The following message is shown:
"You cannot change the products type because it is already used in sales orders."
However, we can close the message and save.

Problem:
If some sales were already made, it should not be possible to change the product type.
There is a warning message on the onchange but it's not blocking.
This causes inconsistencies between the quantities and value shown in the quants and in the valuation layers.

Solution:
Raise an user error when trying to save the changes.

opw-3000886

closes odoo/odoo#101547

Signed-off-by: William Henrotin (whe) <whe@odoo.com>