Working on a how-to to change the odo when swapping gauges.....stand by.
Here's a pic that should help. I like to use a box cutter or knife to slide it up up the shaft alittle bit. Then take a fork to her. BUt....Keep in mind.....you'll need to recal your speedo.
And to do that, you'll need to make two Y-wires with aligator clips/roach clips clipped to the little solders on the outside of that speedometer round silver thingy. the other end to a AA battery. Doing this should make the speedo read about 77mph, but it may vary. Do it a couple times, to get an average, then pull the needle. Do whatever you need to do, then connect the battery again, and have an assistant place the needle backon the shaft to that previously found out cal number.