@Darren ********
this is an exception from the rule, called "change of visa type", and only (!!) possible on Immigration inside Thailand. Not possible abroad or in your home country. Refer to the different "changes" on this Immigration website. The DTV is not mentioned under number 9, but an Immigration officer might accept it as the "tourist visa" from out of which he might allow you the "change". Check number 9, it refers to the "90-days Non-Imm-O Retirement visa"
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