This is the absolute worst time to be considering doing this. You're already in the period (2 months before your extension) where you have to be able to show your account is at 800,000 baht. If you want to change banks, you should do that in the period where your account is only required to be at 400,000 baht. This would be 3 months after your extension, until 2 months before your next extension. That's 7 months where it makes sense.
During that period you can transfer 400,000 and let it settle for a few days, showing you meet the 400k requirement. Then transfer the other 400k. Then you know you'll be covered since you met the 400k retirement with 2 accounts, and you'll be prepared for the next 800k requirement.
Any change of bank will 100% require all documents from both banks. But trying to do it right now is putting your extension at risk.
What documents do you need translated for a retirement visa? That doesn't make any sense. Nothing that's required for a retirement visa/extension would be in a foreign language.
You could, but why would you? Are you planning on leaving 4 times during your 90 days? That's the only way it would make sense, because a single re-entry costs 1000 baht and a multiple re-entry is 3800 baht. A re-entry permit is only valid until the end of the stamp you purchase it on. So if you're on 90 days now, the re-entry permit will only be valid until that ends.
You can purchase a single re-entry permit at the airport on your way out. After you get the annual extension, purchase the multiple re-entry from immigration.
Depending on your immigration office, you can apply for an extension when you have 30 days or less left on your stay. At some immigration offices, you can apply when you have 45 days or less left. That means the only chance you would have to apply for a 1-year extension would be during the period you will bein Thailand for 60 days, and then only if your office is one of the ones that allows you to apply when you have 45 days left on your entry stamp.
Your visa is like an entry ticket. The ticket expires 30 November, and you can use it to enter Thailand until that date. When you use it, you then receive the entire permission to stay that visa grants (60 days) starting from the day you enter.
Embassies normally don't care where the money is at. You can use money in your foreign bank account. But once you're in Thailand they only recognize money in Thai bank accounts.
The non-O is the 90-day retirement visa that most people use to get started. Very very light requirements outside of the financial ones.
The non-OA is a 1-year multiple entry retirement visa that can ONLY be obtained in your home country. It has much higher requirements including mandatory insurance, criminal records check, health check and whatever else your embassy wants to add. If you start on this visa, you'll have to keep the insurance forever.
You cannot get a non-O visa and then apply for the non-OA, and especially not while you're already in Thailand.
You need to apply for the non-O visa from your Thai embassy using the e-visa system before you leave for Thailand. This visa will allow you to open the required bank account. If you're going to be arriving in Thailand next month, you can apply for the visa already since you can use it to enter Thailand for 90 days and you're already in that window.
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Once you arrive in Thailand you will want to open a bank account ASAP using your visa and possibly you'll need a certificate of residence (COR) from immigration. You'll have less than a month to get your bank account opened and the money transferred, as it needs to be in your account for 2 full months already when you apply for your 1-year extension. Since you'll only have 90 days to get this all done, you can see how tight the timeline is.
The non-o IS the retirement visa. Get it from your Thai embassy before you leave the UK. Then when you arrive in Thailand, get your bank account opened ASAP because you need to transfer your 800,000 baht in there so that it has enough time to sit for 2 full months before you apply for your extension. That means you only have a couple of weeks to get it done if you want to apply before the last possible minute.