I recently bought myself an akg-k420 recently for cheap on catchoftheday.com.
The stock ear pads were relatively comfortable but after a few hours I needed to take a break from them.
Just for some experimentation I did pull apart 1 of the ear pads, which is glued to a plastic O ring.
The glue wasn't strong enough so the ear pad came off without the plastic ring.
So I put used a plastic card to stuff the edges back in, and unfortunately the end result was a misshapen ear pad.
Not content with the results, I decided to replace them with the Sennheiser HD-414 ear pads.
They fit quite well and are a lot more comfortable.
There is no significant change in sound quality but I did notice a hint of a slightly more open soundstage as the padding in the centre of the HD-414 pads are thicker than the akg-k420 stock pads.
For the other ear pad, I figured out a way to remove them along with the plastic ring in one piece.
First you will need to get a small screwdriver and feel for the screw holes, lift and hook it over the lip.
After that, you can repeat this in a circular motion.
Please not that the plastic ring may tear during this operation so take care to do it bit by bit.
I did find a disassembly video on youtube:
http://www.youtube.com/watch?v=WNBjuiOrxGw
I may post the photos up on head-fi when my account gets approval to do so.
Happy Listening,
H.M