So do you feel the same way about all the Thais who apply for and pay for visas to other countries, walk into the interview, and are denied without barely a word and no reason given?
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.