Can I top up my retirement visa account balance from another Thai bank or cash instead of transferring from abroad?

Make sure you don't confuse the timing of bringing it back up to 800,000. It has to be back to that amount a full 2 months before you APPLY for your extension, not 2 months before the extension expires.

I always appreciate Tod's clear and concise comments.

I believe you are confusing the seasoning requirements for the 800K baht banked money method..

the 800K baht has to stay in your account for 3 months after you get the extension granted then the balance can't go below 400K baht the rest of the year, until it goes back to 800K baht a minimum of 2 months before your next extension application.

There is no requirement that the banked money method funds come from abroad for yearly extensions, it can be a domestic transfer