The bank in Ranong should be able to produce the letter and statement you need for the conversion/extension. However, as there are some things they will not be able to do - new passbook, new ATM card, etc., it would be worthwhile trying to open an account with them. It costs nothing to do it and if they will without too much foolishness it's better all round. If they insist on you needing a non-O visa to open then go ahead and get the documents first then you can go back once you have the visa and open an account.