Just got an email from MPC that my claims for China Eastern miles got rejected because it is R class. I didn't realized I flew R until this email because I bought the original ticket with M class (50% miles), but I did change the date with $100+ USD of penalty. I would assumed it would be the same class, but now I know my assumption is wrong.

Just wanted to ask is this standard practice in airline industry? On a changed date ticket they can lower your ticket class?