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I've been living in Thailand for the past couple years, and have exchanged money for work and travel at quite a few banks - I've never had problems, though. If you go to a bank, make sure you bring your passport, and prepare to wait in line for a bit. If possible go early to avoid lunch break and post-lunch food coma. I'm not sure about any money changers in Thoeng, however I would imagine any travel agency or larger hotel could point you in the right direction. If you go that route, check the exchange rate first, I would recommend the kasikorn bank website Good luck :)