Considering even if there is life on Mars it's going to be a long long way from Little Green Men I don't see why anyone except creationists would flip their shit. And even then, creationists and cdesignproponentists will ignore it and do the fingers in ears na-na-na thing. So nothing really would change except smart people would redefine their picture of the universe.
Even at that, considering how much material Earth and Mars have exchanged over billions of years, it wouldn't even really be that amazing for single cell life to be on Mars, especially if it has a common origin with life on Earth. If we proved beyond doubt that it had an independent origin, THAT would be big.