I think the dangerous part would be altering the planet to be unlivable to the current inhabitants. Which is a good reason for conducting these tests, to see if anything is there before we start transplanting life. If nothing is there, then we can move lichens etc without fear of contamination. If something is there, we don't want to destroy its habitat by introducing new life.
Ive just realised how good AND bad finding life on Mars could be. On the one hand, we find LIFE on another planet. On the other hand, it makes it incredibly difficult for humans to ever move there and begin terraforming the planet into a second home.
