Where to change money in Bangkok Suvarnabhumi airport

 The best place to exchange money at Bangkok Suvarnabhumi airport is downstairs by the Rail Link station.

Once you collect your suitcase and exit to the arrivals terminal, follow the signs to the Airport Rail Link

These signs will take you downstairs, to the lowest level. You are on the second floor, so you'll need to go down two floors.

Once you reach the entrance of the Airport Rail Link

Instead of going to the Airport Rail Link you go through this area highlighted below

You'll go through a bunch of little shops right and left

Just keep going all the way to the end

the shop you are looking for is the one in the picture above (SuperRich). Usually it's the one with the longest queues.

Don't be deceived by the prices you can see on the other shops, they may look the same or even cheaper that SuperRich but it's just a rouse to bring you in, once you exchange the money they give you a much worse rate, if you complain about it after queuing up for a while, they'll offer you to change it back...but by that time you already wasted enough time and find it hard to go back to the other longer queue in SuperRich. 

The difference is probably about 2% more in the other shops, depending on how much money you are trying to exchange, that may be an acceptable extra charge to avoid the queue in SuperRich.