If you have an old Maglight, by all means put the new LED bulb in it. It's brighter, and your batteries last longer.
As someone else said, the $14 price is probably a closeout to make room for the newer, slightly brighter version. If you are happy with your old light, you will be even happier with the $14 LED upgrade.
The Maglight LED bulbs are fatter than a standard flashlight bulb, so they won't fit in a standard flashlight.
The Maglight bulbs get dimmer as they get hotter, to keep them from frying themselves. This is good, except that the Maglights are the worst possible design for dissipating heat. So, your bulb starts out bright, and gradually winds up less bright after a little while. Of course, it's still brighter than the old incandescent bulb.
Don't buy a new Maglight. You can get better lights cheaper, and much better lights for more money. Here is one place that has the good stuff, and is really great on customer service:
lighthound.com