I am presently a teacher here and have a one-year visa and work permit (expiring next year). I have placed 800 K Baht in my bank. Will I be able to change my current visa to a retirement visa in a few months, without leaving the country? Thanks

I assume you have an extension of stay based upon working you applied for at immigration (it is not a visa).

You can change the reason for your extension of stay one based upon retirement. The 800k baht will need to be in the bank for 3 months on the date you do the change.

You will need a termination letter from your employer to cancel your current extension on or shortly before the date shown on it so you can apply for a new extension based upon retirement.

No need to leave the country.