Once the hair grows in, it's there permanently unless you were to get it permanently removed via laser or electrolysis. But it wouldn't grow into new areas after stopping.
So if for example you were to take T for a year, and grew longer sideburns and a goatee but no connecting hairs along the jaw line or a mustache - you'd still have the sideburns and goatee after stopping T, but wouldn't have to worry about a mustache or jaw hair coming in.