I don't care for the global warming slant but the theory and science has been there for years to include, theoretically, changing the trajectory of a comet and basically smacking it into a quality planet to produce a man made " big Bang", erupting the planet and letting the natural forces to do its job....and wallah... an M Class planet emerges.
That is a layman's version of course.
The problem won't be ethics, what is the difference between choosing whether to disturb microbial organism and aborting a pregnancy? People do it without a hitch these days. If they weren't going to merge into some higher form of species then they would stay a jelly/omeba/germ. I think Ethics would come into play if we actually found sentient life beyond a microbe such as actual animals or if you will, humanoid beings.
No the big problem would be getting people to go along with the attempt at terraforming because when ever anyone starts this conversation, the horde wants to cry and complain about problems at home, welfare, and repetitive social issues, war, politics, and nonsense squabbling. I'm afraid this would have to be a world effort, gathering the best minds from the leading industrious nations to reach a common goal of reaching mankind beyond earth..... and that is a huge, huge, order.
You know if people took more effort and excitement into the sciences than how many more inches Kim Kardashian's butt had increased, we'd be a very highly sophisticated people.