I managed to upgrade the best-selling Koss PortaPro headphones using a couple of items I've bought on eBay. The headphones are some ten years old now, yet sturdy and rocking, and now have become even better:
Enhancement number one was replacing the original earpads with new ones, labeled Luxury Replacement Ear Pad Cushion for Koss Porta Pro, not quite leather, but alike. These look and feel much better:
The procedure is pretty trivial: you detach the earcup from the headband by gently pulling it out, take the old foam earpad off the driver and put the new one on.
The second enhancement was upgrading the original oldschool cable to a fancier one, with player control buttons and a mike, thus turning the device into a headset (you can use the search phrase DIY Headphone Wire Cable):
It works perfectly with my Samsung Galaxy Note 4, however the Rewind/Fast Forward buttons change the volume instead. Which I don't mind at all as it's exactly what I need.
The procedure here is but a bit more complicated because it involves some soldering. Likewise, you detach the earcup from the headband and remove the cap protecting the wire leads by gently lifting it with a screwdriver. Then desolder and remove old leads and solder the new ones, observing polarity (minus on the left, plus on the right) and channels (left channel must be connected to the tip of the plug):
Finally replace the lead cap and voila!
I've only managed to find a read cable, but it's kind of trendy nowadays.