> the exchange of emails and payment sound like some kind of contract
Beware that this is very dependent on jurisdiction. There are places where such informal agreements are completely non binding. IP laws vary quite a lot as well. So your intuition might be completely wrong. Just sorting this out would be a nightmare (the author was working in Germany for a Swedish company; which law should apply? Then there is Microsoft).
> If Microsoft replace the poem, I'm not sure anyone would notice except the author wouldn't have a fun fact for his bio.
He would still have it: this happened and whatever happens next won’t make it un-happen retroactively.