Processing constraints : how to get the old value back ?

Hello,

We would like to use processing constraints on sales order form to prevent users to update fields like customer (which is seeded from quoting).

This processing constraint raises a customize error message saying that this update cannot be done.

However, the value entered by the customer is still into the field and he has to change it manually by exiting the form.

Is there any way using these constraints to put the old value back in the field.

Example :

Customer A is seeded from quoting.

User changes Customer A to Customer B.

A message is raised to say that this field cannot be updated.