I have used Thai Visa Centre several times over the past years and was happy for their honest, fast and excellent service. THAI Visa Centre is definitely NOT a scam.
Every immigration office has its own rules. In Hua Hin I just needed my old and my new passport and I had to fill on a form. That's all. Of course, I also needed to pay a fee. I think about 500 baht. The whole process took about half hour.
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