The LNB slides out of the arm. Its held in place with two screws. If the new lnb matches the old one where it slides in, it will work physically, however, the shape of the dish is designed for each type lnb. The one with 3 horns is for an ellipital dish (phase 3) and wont work on a round 18" dish even if it fits.
If your concerned, there are lots of people upgrading to HD, and discarding their old 18" and phase 3 dishes. Just hunt through craigslist, or ask your local satellite dealer to bring you one back from an HD install for a couple bucks, get one on Amazon, or Solidsignal.com. Lots of options there.