Do you have to change all pistons if only 1 is bad?

I HAVE NEVER CHANGED PISTONS IN AN Audi, BUT IN AMERICAN CARS YES AND WE HAVE HAD PLENTY OF BAD PISTONS AND WE DIDN'T ALWAYS CHANGE ALL OF THEM, ONLY THE ONES THAT NEEDED CHANGING. ANOTHER FACTOR IS BASED ON THE MILEAGE ON THAT (ENGINE). MY BEST GUESS IS ,IF THE ENGINE HAS LESS THAN 100K MILES CHANGE ONLY THE ONES THAT NEED CHANGING. UNLESS ITS MORE THAN HALF OF THE AMOUNT OF PISTONS IN THE ENGINE, CHANGE ONLY THE BAD ONES, IF YOU ARE DOING IT YOURSELF. A GARAGE MAY WANT TO REBUILD THE WHOLE ENGINE, MAINLY FOR WARRENTY PURPOSES. GOOD LUCK. I HOPE THIS HELP SOME.