I had to change passports (old one was full) and it's hit and miss. Mostly I just hand the print out of my DTV and if you smile and are polite they'll be obliging. But every now and again you get someone (Thai IO) who's either having a bad day or just looking for a fight, and they'll make it difficult. But having re-entered into Thailand on my DTV at least 20 times I've always got through with just the new passport. There is no facility for Thai Immigration to transfer the existing visa to the new passport number. Seems a blatent miss by them (to me), but it is what it is.