check out the review at inner fidelity, they do a good comparison between the old and new. bottom line, the m50x DOES sound different, even though AT claims they haven't changed anything.
some on this forum have noted that the older m50 sounds different to newer m50, and it was more than just wear. seems AT tweaked the m50 headphones over time, so perhaps the new m50x sounds the same as the newer m50, but if you have an older one, the m50x should be quite different. whether you prefer one or the other depends on your preferences, would suggest AB testing them at the shop.