Point by point replies follow.
Hello. I am a new HO modeler, and I have a newbie question. I've noticed two different types of couplers on HO equipment. Could someone explain the difference between them, and the advantages of one over the other (if any).
At left is a "knuckle" coupler made by Kadee. Kadee made the original design of this type of coupler; there are now all sorts of knock-offs, but the Kadees are cast in metal and still the best. They also have a large number of variations for specific mounting situations, while most of the copies only have one version, a copy of Kadee's #5.
At right is an older "horn hook" (sometime also referred to as a "NMRA" (although to my knowledge it was not actually accepted as an official NMRA standard coupler or anything) or "X2F") plastic coupler. These were common on just about all rolling stock from about 50 to 10 years ago. (ish. plastic copies of the knuckle coupler are now standard, and for a while (I think manufacturers are now currently no longer doing this) cars came standard with the knuckle coupler installed, but the older horn-hook couplers included in the box to change out if desired.)
While not _exactly_ looking like a real coupler, the knuckle coupler has the advantage of looking more like a real coupler than the horn-hook, has a metal trip pin that enables magnetic uncoupling.
The horn hooks used to have the advantage of being dirt cheap and simpler to install; they're harder to get now - I'm not sure anyone even makes them anymore, but there should be scads of used ones out there (although most modellers probably just throw them out when replacing them with knuckle couplers, you can sometimes get them for cheap or free at shows and swap meets.
Also, are they compatible with one another?
No.
Can I change one type out for the other, so they are all the same?
Yes.
Since they're not compatible, yes. It would be best to have all your couplers of one design.
Some people run "conversion cars" with a knuckle on one end and "horn hook" on the other to connect cars of different couplers. This of course limits your trains to running with a pretty fixed set of cars in a fixed order; this may not matter to you, but if you ever want to do any sort of switching or "operations" then this is a no-go. I expect to be able to match ANY cars and/or locomotives with each other on a layout.
How difficult is it to do that?
Varies. Most newer cars are a 30-second job. Remove the screws holding the coupler box covers on, remove the old coupler, put in the new coupler, screw the covers back on.
Cheap older cars with truck (wheel and axle bearing frame/assembly)-mounted couplers will be the most difficult. Either cut the extension and coupler mounting away from the truck and attach a new coupler box to the body/floor, or you'll need a special truck mounting conversion kit.
Are replacement couplers readily available?
Yes.
http://www.kadee.com
Their #5 is their "standard" coupler; that's what's pictured on the engine at left in the photo.
The #58 is equivalent but with a smaller, more scale-size head.
Everything else is a variation to handle mounting situations where a #5 doesn't fit or line up properly. Try a #5 (or #58) first, and if it doesn't work, then look for a different version that will.
There are other copies, mostly cast in plastic (Bachmann "EZMate", McHenry) but the Kadee is still the best.
Photo shows two of my locomotives, with the different types.
(I know, lots of questions. Sorry 'bout that, but I want to learn.)