If the planet does have a sentient species that isn't too established, I can see us moving in, setting up camp, and altering the atmosphere in slow degrees, all the while making excuses about how it's not all that lethal for them. Then we set up special relocation programs to move them all to some crappy planet somewhere.
Why would we do that though? If we can alter the planet's atmosphere, then we could surely just go setup home on another planet. It's not like there aren't enough goldilocks planets about anyway.
I mean, there's what... Four? Well, Three at least, in this system. Going to another solar system then saying "No, we want THIS planet." seems entirely illogical to me.
Because we're humans, and humans are amazingly short-sighted, self-absorbed, dicks who will almost always take the way that looks financially/economically/ideologically easiest but causes everyone involved amazing amounts of long-term suffering and sets us up for generations of trouble. Finding easy-to-terraform planets is pretty costly, I imagine. If we found one with aliens on it I don't think we'd hesitate for a second (as a whole) to take it over and make whatever excuses we could to justify getting rid of whatever's in the way.
Hell, I'm pretty sure that if we came across another species in space, it'd take us a few generations to even admit they were sentient, no matter how compelling the evidence we came across. "That metallic thing they're riding in orbit around their planet? Oh, I'm pretty sure that's just natural for them. The've clearly evolved to create space-homes through instinct alone as a survival mechanism. No civilisation or science here, start the terraforming!"
Actually, in total seriousness, I'm somewhat of the opinion that once we start coming across non-spacefaring alien species (however many millenia that may take), we're going to take a long time to realise that our own interpretation of sentience is only based off our own species, habits and cultures. We'll probably end up torching a planet of hyper-intelligent sentient fungus or something simply because we didn't see any metalworking apes on the surface, not realising that the weird, unpredictable heat pattern we kept detecting on the surface was actually the fungi giving each other lectures (using some form of communication we've never come across) in how to create unlimited energy using hyper-quantum mechanics or something. Seems perfectly in keeping with our collective character so far.