I also went to Chang Wattana recently for the same extension and would thank Steve for this information. I would add the following.
FORMS
Add STM 10 - Affidavit
STM 11 - Consent form
STM. 9 (The penalties form)
I was asked for updated KR2 and reminded to update again next year.
Documents
Original Visa document
Bring all originals and do two sets of copies. I was asked for a second set and she just took what she wanted from it.
BANK
Only needed letter and updated book on the day.
If you haven't booked an appointment get there as close to 0830am as possible, get your ticket and get your paperwork checked at counter 27. This is really helpful.
I was finished by 10am......very happy
Thanks again Steve and hope the extra info is helpful