Update lookup field with a direct sql command

Hello

In the Quote entity, I have a custom lookup field pointing to a custom entity. For some reason, I must update this lookup field when the quote is in active state (readonly) and from an external application.

If I use CRM SDK I cannot update the field because the quote is in read-only state, so I thinking to do the operation with a direct sql update to the database changing the guid in the field.
Is it a good idea use this solution considering the field and the external entity are custom?
If not, have you some suggestions?

Thank you very much

Matteo V.