I'm not sure if to start a bounty or to start a new question in this case: I asked a question about building a tinily modified bitcoin source code and got a plausible answer and updated my question. Should I start a new question referring to the old one or just offer bounties for an updated answer?
Unfortunately, the Stackexchange format doesn't lend itself to evolving a topic in one post. So, if your update to the question changes it so much that answers already given would become obsolete by being wrong, off-topic, or severely incomplete you should rather ask a new question instead.
In this case, I think it could work out, but perhaps you should write a comment mentioning the author of the answer, so he'll get notified and has a chance to update his answer as well.