It'd surely have impact in the related fields. However, I'd save the "biggest discovery" title to findings that debunk and completely change our assumptions, rather than more evidence for assumptions we already hold. Perhaps, having another life origin to study will lead us to new understandings. But the mere discovery of alien life is not that disruptive.

Given all theories and evidences we've observed thus far it's expected that we are not the only living things in the universe. Except for some fringe theorists, everyone expects life to exist in other places besides Earth. Finding out that life exists elsewhere in the solar system would just be more evidence for what we already belive to be true. So, not really huge news in itself.