FWIW, I just replaced the idler tires on my 246 a few minutes ago, and I'm not sure I understand what all the fuss is about. Because of this thread, I went in through the bottom, took out the mechanism and disconnected all the wire harnesses so I could set it on its own.
After reading, "Then you'll remove the 4 screws that hold the portion with the capstan motor and control motor," from fucanay's post above and not knowing exactly what was meant by that, I finally actually looked at the idler tires to see what was going on. It was at that point that I realized I should be able to replace the top one without removing anything. And that was true. For the bottom one, though, I needed to remove the top one.
So I just carefully removed the little retaining ring that was hold the top idler tire down with the help of a jeweler's flat head screw driver . After that, it sprung up a bit because of the spring. I pulled it up, with the spring, and then pulled bottom idler tire right up off the shaft.
I replaced the bottom idler tire and then simply reassembled. You can't mess up the reassembly of the bottom and top idler tires (at least on the 246) because they will only fit one way together. The bottom one and top have opposing grooves, so they fit together like a puzzle piece. The hardest part of the whole procedure was putting the retaining back on, but it only took about 3 tries.
The whole process took about 5 minutes. And, of course, my biggest regret was that I could have done the whole thing from the top.
It needs to be said, of course, that taking off the top of the 246 is really easy. You just remove the bottom cover (about 7 or 8 screws or so) and then remove 10 screws (I think) to free the top cover. At that point, you can access the pinch roller from the top as well as the idler tires, using the method I explain above.
Again ... this is for the 246. I've never owned a 244, so I don't know exactly how they differ.
I didn't think to take pictures of it while I had it apart, but, like I said, you really can't screw it up. The bottom and top go together like a puzzle. And of course be careful removing the retaining ring, because you don't want to lose it.
Other than that though, it was a piece of cake.