You don't mention which embassy or what is the reason for your extension.
My guess is that, IF your embassy is one of the few still issuing them, you will most likely need a new letter (my guess is that you want to extend a tourist visa?)
Nobody knows when you'll be allowed to return. Are you married to thai national? Or father to thai kids? If so, get on to the embassy and get your name down for repatriation flights.
make sure that they know you're flexible and will be available for a last minute cancellation. I've heard of a few people getting a call "can you be in frankfurt tomorrow?"