@Steve ******
You will cancel your work permit at the labor office. You will need the following:
1. The form for the Work Permit Cancellation and two copies
2. The original work permit.
3. If the employer can not cancel the work permit in person with the employee, a Power of Attorney form with a 10 THB duty stamp and a copy of the employer’s ID card. A 10 THB duty stamp can be acquired at most stationary shops.
The extension of stay becomes invalid when the work permit is cancelled, and a person must leave Thailand no later then 11:59PM the same day. The extension of stay must be cancelled, and either a new extension of stay acquired, or an extension of stay denied stamp issued to stay longer.