I usually try to improve an answer first, either by adding links to references or other questions on the site or generally improving it.

However if it ends up that I'm making so many changes that I'm butchering the person's answer I'll just post my own. If someone's post really helped me but didn't quite meet my needs and I post my own I make sure I give them proper credit ([example][1]) so that the question now has a complete solution, but also other information that might be useful to the next person with the problem.

It's really a judgement call on your part.

  [1]: http://askubuntu.com/a/4578/235