Pretty much as the title. I would like to add more information and edit the organization of the answer I gave and was selected by the asker, is it bad practice? Will my answer lose the selected mark?
Yes, go ahead and edit! Make your answer better. We all do it, even after the checkmarks or however many votes arrive. Usually it just means more votes - our answer did get better.
You won't automatically lose your checkmark. The asker may change who gets it, of course, especially if the usefulness of your answer gets eroded by the edit.
If your edit would change your answer very majorly, you ought to check this question: Do I edit or create a new answer when my view on my current answer changes significantly?
Please feel free to improve your answers!
The Stack's a Q&A site rather than a traditional discussion forum: that means it's designed to be a long-term repository for actionable solutions to real problems, sorted by usefulness and quality. Improving the quality of posts is always welcome, no matter how old they are, because we're always expecting people to be looking them up. The mechanics (like acceptance) won't automatically penalise you for it, and you may even get a badge!