You need to cut the fabric off gently and carefully making sure not to clip the wire inside. After that remove the giant massive heavy rubber piece and foam. Start snapping off the other sliding plastic pieces. Re-cover the top with your choice of coverings. Perhaps add a 580/600 headband for comfort.
It's an easy mod, somewhat dangerous but the weight reduction makes it worth it. Also, the shure 840 are so badly designed, they get my pick for the absolute worst and most dangerous design of a headphone of the last few years. The headband is stupidly large and heavy, the rubber piece is so heavy and didn't even need to be there. The wire is exposed on each side, why? I have no idea why as its immensely stupid in its design and placement. So dangerous, as I've recently found out. Mine took a tumble and got caught right on that exposed wire and got shredded.
But they sound amazing :) just a lot of stupid design flaws.
What you mean, when you say “It's an easy mod, somewhat dangerous but the weight reduction makes it worth it“ how come it's dangerous?