It's easy to change the elastics. First remove the type sign (plastic sticker), a needle or a thin blade will do. Unscrew the cap and open. Likewise losen the two screws on the elastic holder on the headband.
The part is available as a spare part, but I just change the elastics on my K400, 401, 500 and 501 headphones, using a sharp knife.
Picures below from a K500, but it's basically the same.
Left aged elastic, right new.