Somebody in the US has bought a c600 casing from me to do just this. You will need to change the moulded chassis as well as the aeriel cover, and I believe the power button pusher is located differently so that may require some modification.
He's promised to do a method if it all works out OK.
I did purchase a c600 casing from Richard recently to fix a scuffed exterior on my 2125. Worked out fine (more or less). A few things to note:
(1) I replaced only the front and the perspex (in the US we call it 'Plexiglas') cover. I didn't replace the back, or the aerial cover since neither piece actually would be a stock replacement for the 2125 housing due to the hump. That was fine for me since the back of my phone was OK, it was just the front that was damaged.
(2) As Richard notes, the power button is different on a C600 than on the 2125. The mechanical switch on the circuit board is identical, however. The c600 power switch is actually an easier way to power-on the phone; after taking apart my 2125 I discovered that the reason the power-on switch on the 2125 is so wacky is because the addition of the hump prevented the switch from being activated using the c600 button. So they (presumably HTC, on Cingular's direction) bolted on the wacky power button to accomodate the hump.
You'll need to manually remove the c600 power button from the front casing, but that's quite easy (just pull it off. The plastic HTC used for the housing is about as solid as model airplane plastic - a bit disconcerting, actually).
(3) There's some contact metal that surrounds the LED housing on the top of the phone. I relocated that metal tape from the old front to the new one to ensure the LEDs would fire appropriately.
(4) It's helpful to use surgical gloves while handling the Plexiglas. Otherwise you'll run the risk of oil from your fingers getting on the wrong side of the Plexiglas, causing display difficulties.
(5) I didn't need to swap the chassis (or even disconnect any ribbon cables).
(6) I used a Torx #6 to remove the screws on the housing since I couldn't find a Torx #5. The screwdriver was helpfully labeled "for use with cellular phones." Took a bit more elbow grease than I expected, but it worked OK although I couldn't remove the additional two screws that would have enabled me to pull out the chassis and circuit board.
(7) Many have moaned the lack of a good case for their 2125s. I ended up repurposing a Nokia case (model number CTU-07); accomodates the hump just fine. I don't care for the plastic cover over the keypad so I snipped that down using a pair of nail scissors, otherwise no complaints. Had I purchased this case earlier, I wouldn't have damaged the front of the phone when I dropped it, necessitating this procedure.
Thanks to Richard and also Bret for their detailed dissasembly instructions.