Koh Chang is essentially a "Cash Society" except at the larger hotels and resorts where credit cards are usually accepted.
In Thailand the banking secor is a law unto itself. Even in the small sub-branches on Koh Chang it is unusual for there to be more customers than staff in the branch at any given time. Also, if you have an account in one province and want to deposit money into your account in another province you will pay a fee and from there the fee structure starts! Be patient and go with the flow and you won't have any problems. If you are planning on having an account for complex transaction its probably best to open your account in a main branch in Bangkok.
There are ATM's all over the island. Several banks have sub-branches in the main beach areas offering basic services including money exchange, money transfers {Moneygram® and Western Union®) and basic account banking. SWIFT transfers are also available. Most exchange places won't accept damaged foreign notes and if you have series 1999 US $100 bills you will have a problem changing them too.
Travellers checks are accepted by larger establishments and can also be changed at the many exchange offices on the West Coast but smaller establishments don't accept them.
If you shop around you may find differences but is the effort really worth the time? Here are the current base rates as a guideline.
But don't pull into the local gas station and expect VISA® to be everywhere you want it to be!