If you want to replace only top outer shell, than it's possible by removing four screws and sliding it carefully out..
However, not all replacement shells are compatible with each other....so you maybe won't be able to put on some other one..

And if you want to replace inner shell of top part of your NDS, you'll have to disassemble the whole NDS..

 

Can I do it with this case: http://www.dealextreme.com/details.dx/sku.14103
I broke my DS when I put this case on and I'm getting a new DS today and don't wanna break the new one but still replace the best part of the case so when I go back to school and show-off

 

That is the brand of cases that I use. They are the best fit you are going to find, but the corners of the top piece stick out a little more than the OEM panel. That piece should fit onto the DS without using any extra force. If it is difficult to put on, you are most likely doing it wrong!

Oh, one last thing. The screw holes on the piece you want to use will most likely be too small to use. Just use something that is larger than the screw hole (but smaller than the screw) to widen them up a little. I have purchased three of these cases and they all seem to have small screw holes.

 

Great!! Have you posted pictures up?! I'd like to see it.