well, but a DTV first needs to be cancelled by the Thai embassy that issued it. Thai Immigration cannot cancel a visa or a stay permit that wasn't issued by them
"change of visa type" as mentioned on every Immigration form, are from two types of "touristic" entries only: from the single entry 60-days tourist visa, or from the visa-exempt entry. Nowhere is mentioned that you can do the "change of visa type" from an Elite Privilege Visa or from a DTV to the initial 90-days Non-Imm-Visa
you might need this account, because since recently, some Immigration offices have started to require a few transfers from abroad from applicants who use the embassy income affidavit. You need to ask your Immigration office what their current policy is
some Immigrations have changed their policies against applicants who use the income affidavits of their embassies. They now additionally require that the applicant has a Thai bank account, and can show a few transfers having come into it from abroad . . . . . The OP should URGENTLY ask "his" Immigration if they require a Thai bank account, additional to the embassy affidavit
not even in this case would a tourist visa whitewash the fact that you used "touristic entries" for a longstay in Thailand. It doesn't make any difference - both the visa-exemptions and tourist visas get scrutinized if used "too many times" in a row
the exchange rate of today doesn't match these numbers, so I would only touch it with a pair of golden tweezers. He can however do the math himself on how many GBP will equal the minimum of 800,000 THb in today's exchange rate
the 90-days Non-Imm-O retirement visa is valid for use for a single entry within three months counted from the date of issue. This will be noted on the .pdf visa document under "visa must be used by" and the date