Filling Redone or Onlay? (photo)

I had my dentist check a stain by an old filling. I wanted to make sure it was not decay. He said, no decay but it looked like the old filling might be failing. Due to the size of the filling, it could not be removed and redone and I would have to progress to a porcelain onlay. Can a new filling have been added right next to an old filling? Or does the whole filling have to redone and refilled? Why does it have to go to an onlay and not stay a filling?Is it risk of having the tooth crack?