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.