After a few months I noticed that the earcups on my m50's had gotten harder. I did some research and found that this was probably caused by sweat (as that is one of the things that hardens plether) so I began looking for replacement earpads. I haven't had much luck so far so I decided to check here to see if anyone else has had my problem, or knows a solution.
P.S. I would prefer velour earcups over plether ones to prevent this from happening again but I suppose I can always be more careful with plether.
It says that it works for the m50s, you should be fine. You could also try (as a means of last resort since you are about to stop using them anyway, might as well try to save some money) using some lotion or something to get the pads to soften up. Many people prefer velour overall though, so that would probably be a good option if lotion doesn't do the trick. So note that changing the material of the pads will affect the sound some. Velour usually makes it sound more open and lowers the bass impact compared to leather, so be aware.