I have just moved from Kanchanaburi to Bangkok. I currently have a Tourist Visa and want to change that as soon as the funds have been in the bank for 3 months.  My bank account is in Kanchanaburi, I cannot open a bank account with the same bank in Bangkok as I don't have a work permit. My question, do I need to go back to Kanchanaburi to get the letter I require?  Thanks

well first thing is you can't get a retirement extension on a tourist visa. With a Non O you can open an a/c easily and you need one for the retirement extension anyway.

Does not have to be a 1 year, 3 month will do

You will have to apply for a 90 day non immigrant visa (category O) entry at immigration before you can apply for an extension of stay based upon retirement. The money does not have to be in the bank for 60 days to apply for the visa. You  will need to have at least 15 days remaining on your 60 day entry or the 30 day extension of it to apply.

After you get visa you can apply for the extension during the last 45 days of the 90 entry you will get. Since it will be your first extension the money only has to be in the bank 60 days. It will need to be in the bank for 3 months for your 2nd extension.

You should not have a problem getting the bank letter at a bank in Bangkok. Most major banks have branches on the ground floor of Chaeng Wattana immigration.