If you dig around on Google for the Thai Nationality Act, you'll find a Thai language version. I read through once, and it's actually rather vague and no more helpful than the translation. There are, for example, no definitions on what "using your old nationality" means. Better play safe and not use it at all.
For those naturalised Thais who live abroad, there's also the worrying clause that naturalised Thai nationality can be revoked if a total of 5 years are spent overseas. The clause makes mention that the five years is a accumulation of all tge overseas time, rather than a single 5 year stretch. There's no evidence of this being enforced, so far.