I recommend not exchanging money in your home country before travelling to Thailand, as most countries offer poor exchange rates, resulting in significant losses.

When you arrive at Suvarnabhumi Airport, just after immigration, where you wait for your luggage, you will see some exchange counters. They have very poor rates, so if you can, wait to collect your luggage and you will find better rates on the lower floors of the airport.

(luggage claim exchange counter - Poor rate!)

Meanwhile, if you need a prepaid SIM card, you can find providers in the baggage claim area or after passing through customs. They offer similar plans. I recommend AIS, which has good coverage.

Once you have collected your luggage and passed through customs, go to the B Floor (Airport Rail Link).

On the B Floor, follow the signs to "CURRENCY EXCHANGE."



Then you will see a few currency exchange counters. I recommend SuperRich, which has very good rates.
You can actually find the best rates downtown, but it will cost a few hundred baht on taxi fare and, more importantly, more time wasted in the traffic. SuperRich has the best rates at the airport, often better rates than those on Koh Chang, and it's easy and fast, what is very important when you are on holidays and want to relax.

You will always need your passport to exchange currency and also if you buy a prepaid SIM card.

Once on Koh Chang, there are many ATMs (be careful, their rates are not interesting and they add a commission for each withdrawal), but there are also many banks and exchange desks.

Visit this website to see the current rate!

(ATM and changing office from Kai Bae. They are not all like that!)