|I will not be held liable if you try any of the things mentioned below, also note that you will probably void any kind of warranty for your headphones, hands free and cell phone if you do.|
Petterekman at Howard's forum put up some pictures on how he modified his K750i headset using "koss the plug" (link to his pictures).
I decided to do almost the same thing to my headset, but using the Sony fontopia MDR-EX71SL instead.
I started out with opening the hands free. This was pretty easy; I just put my nail in the crack on the side of it and twitched a bit, until it opened.
I started with removing the earpiece with the short cord. This was really easy, what was more of a hassle was pulling the new cord through the grey plastic hole; this probably took me ten minutes.
Soldering the new earpiece on went ok, I am not that experienced in soldering so I was kind of happy to see that I didn't destroy anything.
Now for the more difficult part, I started with removing the plastic that was wrapped around the two green cables. Then I used the soldering iron to remove the green cable that came from the earpiece. Cable number two from the earpiece was blue and soldered with the copper cable on the spot to the lower right; I removed both the copper cable and the blue one from the board.
This time I used a toothpick to force my earpiece cable through the grey plastic; this worked a little better and took me only five minutes.
For the part of attaching the new earpiece, I just twinned the red cable to the green one, used my soldering iron to merge them together, then twinned the two copper wires, merged them together and soldered them to the board.
Before putting the case back on, I put a bit of Dutch tape on the cables instead of the black plastic removed earlier.
I had a bit of trouble fitting the case back on because one of my cables got a bit long, but in the end I managed to fit it in there.
The result was pretty good, the sound is clearer with my new headset and these earpieces fit my ears better.