I wanted to change the backlight LEDs from amber to blue, red or white. Red would be good so as not to mess up your night vision. But I didn't have any LEDs that would work at the drive voltage being supplied. :( By this point I had documented how to take the device apart, so I thought I might as well put it up here.
If you are planning on changing the backlight, you will need two 3mm round (or flat / smd) LEDs that will shine brightly at 1.95V.
Taking the etrex apart and putting it back together again was surprisingly easy, but a little painstaking. Looking at mine now, you can't tell it's been apart.
Step 1: Dissassembly - part 1
Take off the battery cover and batteries. Remove the rubber surround by pulling. It's stuck on with glue. Start at the top where the serial connector is. Don't pull too hard, it's one piece. Place it to one side to make sure you don't get anything stuck on the glue, you can reattach it as it is when you are finished.
Step 2: Dissassembly - part 2
The top and bottom parts of the case are sealed by two layers of tape. The first layer starts and finishes at the bottom of the device. Use a small knife to lift the end up and peel it off. I stuck the tape on the end of my desk to keep it safe and so it couldn't stick to anything else. You need to reuse the tape as it's shaped to fit the profile of the device.
Step 3: Dissassembly - part 3
Remove the second layer of tape. This layer is thicker and not shaped. It has a better glue, but also stretches, so don't pull too hard as you peel it off. The join was on the side of my device. Again, I used a small knife to lift the edge.