If there's life there, what are the chances that the microbes that we send there will be better at living on their world than the nativre stuff is? Sure, some of the native life might die out, like when an invasive slug or fish or plant drives out an indiginous one, but ALL life? No way. Grey squirrels may have driven out the red squirrels here in the UK, but they aren't threatening any other life. Most likely the Earth life will cause some damage, and then crash and die out, and the native life will be back to normal in a few hundred thousand years.