For the life of me I can't find anywhere on our e-tix, nor on the web site, anything about change fees. I'd like to change to an earlier flight for one of our legs (Chiang Rai to Bangkok). Anyone have experience?

Not sure if you are in the US or not but you could try calling them - they are on the west coast. Try 800-426-5204.

As Craig suggested, just give them a call to change, there should be 'no' charge to change a flight.
Thai only has 3 fares, the J Class is Executive Class and fully flexible, the Economy/Coach are either Y or M, the difference is that the Y fare is valid for 1 year ( or it could be 6 months ) and the M class fare is valid for 6 months ( or it could be 3 months ) which is slightly cheaper but the only difference is a shorter validity.

So unless you have a very special kind of fare like the Pass tickets there should be no charge at all.

I have never ever paid 1 baht to change a domestic flight with Thai.

Give them a call.

The Asian airlines are so sensible about such things. Our US airlines could learn something from them!

I just changed my flight from Bangkok to Chiang Mai on Thai Airways -- had no problem and didn't pay a dime.

thanks everyone! ok, i'm getting ready to call and i notice that our legs are primarly on an "AB6" carrier. i have no idea what that is or if it makes a difference as to seating. do you?

AB6 probably stands for Airbus A300-600 widebody aircraft. There are several versions but all are 2-4-2 seating in economy. You can check out the details on the website.

Worked like a charm! I have never heard of "no change fees". Wonderful!

Good. You will also find that once in Thailand if you want to change just go to any Thai Airways office and they will change for you without charge.

I have always found domestic flights within Thailand very easy and stress free. As Craig said the AB6 is wide-body aircraft so quite comfortable, most major routes within Thailand with Thai are wide bodied aircraft otherwise 737s on other routes, but Chiang Mai, Chiang Rai, Phuket etc nearly always wide bodied Airbus 300 or 330 or 777.
The price difference to upgrade to Executive Class is not a lot, you really just get the lounge access and bigger seats/more space.
Don't forget you can accumulate mileage on your Thai flights for Star Alliance ( e.g United ).