You can apply for the "change of visa type" from a tourist visa or a visa-exempt entry, to the initial 90-days Non-Imm-O retirement visa, after you arrived, on Immigration inside Thailand.
The requirements are posted on the Immigration website under Number 9
click onto it, a .pdf list of requirementns will open.
The most important requirement is a bank account, a deposit of a minimum 800,000 THB already sitting in it, and a proof that the money came from abroad. And you will need a rental contract
The fee for the application to the visa is 2000.- THB. It will be issued for 14 days “under consideration”. After this waiting period, you visit Immigration and pick up the stamp.
As soon as the 800,000 THB have seasoned in your Thai Bank account for 2 months, you can apply for the “1-year extended stay permit”.
The “1-yearExtension of Stay” is not a “visa”. It is not a “retirement visa” or a “one year visa”. The 1-year Extended Stay Permit is a simple STAY PERMIT, and that’s what the stamp says!
The fee for the application to the 1-year Extension of the Stay Permit is 1900.- THB
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Here find a report from somebody who did the change:
STEP-BY-STEP DO-IT-YOURSELF INSTRUCTIONS -- APPLYING FOR RETIREMENT STAY AFTER ARRIVAL IN THAILAND
(Must be over age 50):
1. You may arrive either Visa Exempt (currently 60 days), or on a 60-days Tourist Visa.
2. Then, immediately after your arrival, you must promptly open a Thai Bank Account, and transfer 800,000 Baht into Thai Bank Account (if married, some Immigration Offices like CM, accept Joint Account with 1.6 Mil. Baht. Also Note, that either Passbook Savings or Fixed Deposit is acceptable, but not Investment Account. Finally note, that some Immigration Offices, but not all - e.g. CM, require proof of foreign source of funds. WISE is an inexpensive, easy & reliable service to transfer funds).
iii. Then, you apply for a 90-day Non-O Visa at local Thai Immigration Office. If on Visa Exempt, you use Form TM87, if on Tourist Visa TM86. You are required to apply with minimum 15-21 days remaining on permitted Stay, depending on Office (Chiang Mai, requires 21 days).
Requirements: TM87 or TM86 (Note: Fear not, only 1-page Form simple to fill out in English with only basic info., name, address, passport info., arrival info, and stated purpose: "For Retirement"),
Must be Over Age 50, 2000 Baht, Copy of all relevant pages of Passport (i.e. Facepage, and Entry Stamp); copy of TM30 (that is the required arrival form that is either completed by your Hotel or Condo, or you do yourself at Immigration Office within 24 hours of arrival); copy of proof of residency (Rental Agreement, or Hotel Reservation -- some offices may not allow if only staying at a hotel); and most importantly proof of the money in Bank (Copy of Bank Book, and a certified Letter from the Bank usually costing 100 Baht).
Above is all simple and straightforward, so no need for spending money on a Visa Service.
iv. If all docs. in order, Immigration Office will put a Sticker in your Passport that you are "under consideration", and a date to return to pick up your Visa Stamp (prior to your Stay expiration).
Next, when you have 30 days remaining on the Non-O (and your Bank Money has "seeded" 60 days -- i.e. stayed on deposit), you go back to Immigration Office to apply for the 1-yr Retirement Extension.
Requirements.: TM7 (Req. for Extension of Stay); 1900 Baht, and all other same docs. as above (newly updated of course). And, you will receive 1-yr Extension of Stay -- usually same day, if you arrive in morning.
Note: For both processes above, your Bank Passbook Balance must be updated same date as application, and the Bank Letter should be no more than 7 days old -- easiest just to do both same day just before going to Immigration Office).
Finally, you should/must get a Re-Entry Stamp -- easiest/best to just do immediately after you get your 1-yr. Extension stamp. It can be single Re-Entry for 1000 Baht, or 3800 Baht for unlimited multiple Re-Entry. The Re-Entry Stamp allows you to leave and return to Thailand, without invalidating your 1-yr Extension. If you fail to do that, then your Extension of Stay is nullified, and you must start the process again.
(IMPORTANT: If you want or need to leave Thailand while on the initial 90-Day Non-O, same same, you must get a Re-Entry Stamp) Hope that is all clear.