one month is only just enough if you manage to open a Thai bank account in the first few days and have a minimum of 15 working days (24 days) left on your "admitted stay" stamp. The requirements for a change from a visa-exempt or a Tourist Visa, to a 90-days Non-Imm-O retirement visa, are clear, and they require banked money (800.000.- THB or an income affidavit of monthly income of minimum 65.000.- THB) and a lease or rental contract. And of course a proper TM30. It is all listed on this pdf. document from Immigration:
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/9.FOR-RETIREMENT-PURPOSES-50-YEARS-OLD-NON-O.pdf