It's pretty easy, the main thing is you need to remove the motor and add a strip of electrical tape to isolate the motor from the chassis. I can't think of the name right now but I know one of the Kalmbach "How To" books on DCC uses an Athearn blue box engine as an example, showing you step by step how to install the decoder.
Digitrax makes a "DHAT" harness that is designed for converting an Athearn engine to DCC. (Even though it says it's a "no solder" method, I'd probably still solder the connections anyway.) I've used it on a couple of F7's and it's worked fine.
Or just look at the instructions and duplicate the DHAT harness yourself for free. You just have to solder the grey to the bottom motor clip, clip off the tab that sticks out to contact the frame, put a layer of tape in the bottom just in case, solder the orange to the top clip, the red to the tabs on the trucks, annd drill and tap a hole for a brass screw in the frame for the black wire. Cut off most of the clips on the trucks to make room for the speaker and enclosure. For a basic motor decoder in AThearn this should take a few minutes to do, really. A little more to fit a speaker and enclosure, which is the hardest part of the install. The only difference with the DHAT harness is the black wire clips to the headlight bracket instead of a screw in the frame, but this is highly unreliable as the headlight clip is just riveted to the frame is is often loose.
ANd yes, check curent draw, if it's the black motor forget it, it will draw too much power and will need to be remotored. If it's the gold color motor, it will probably be ok, but test. 12 volts DC, and grab the flywheels to hold the motor from turning. The current must be below the decoder's limit or it will fry.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
When I rewire or remotor an Athearn, I solder wire directly to the brass rivets on the trucks (remove the sideframes first so you don't melt them). Use 22 or 24 gauge wire. Pre-tin the wires and put a little flux on the rivets.
Jay
C-415 Build: https://imageshack.com/a/tShC/1
Other builds: https://imageshack.com/my/albums
I agree with Randy in that the hardest part of this is going to be the speaker(s). Have you already decided on type and location for it(them)? An Athearn U28 is one loco not in my stable so I can't pull one out and make suggestions. I could only guess.