See link above. I just did with your post what you can do. The way I did it was:
1. Go to the old post you want to copy.
2. Press on the quote button that also opens the reply dialog box.
3. Copy everything in there, without typing anyting.
4. Make a new thread.
5. Paste what you just copied into the new thread.
6. Add your new text, might also be a good idea to give some background to the post you copied, for context purposes.