for the "change of visa type", you need a proof that the funds came from abroad. You need a rental contract, and you need a minimum of 15 workdays left on your entry stamp, in order to be able to apply. Your case is number 9 on this .pdf list of requirements to the "change of visa type" on the website of Thai Immigration, however I would visit "your" Immigration and ask them for THEIR list, as it could differ a little bit from the official one . . .open this link, change the language to English and dowload number 9
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