Thailand Visa Re-entry permit
If your visa has been extended to one year but you wish to exit the country before your visa expires, your visa will be cancelled. This can be easily overcome by getting a re-entry permit. You can get a re-entry permit at any Immigration office where you need to fill out a simple form and attach a photo. A singe exit re-entry permit costs 1,000 baht and a multiple exit re-entry permit costs 3,800 baht and these are usually processed in an hour or so.
If you exit the country and forget to get a re-entry permit, then the visa is cancelled and you have to reconstruct it again right from the start!