Consumers now have to be extremely careful these days when they want to do mileage upgrades from Y (amongst other things like increased miles).

In the past, if your Y fare class was M or above, you'd be eligible of mileage upgrade. These days you've actually got to pay attention to whether your fare is considered "Econ Light", "Econ Essential", or "Econ Flex". Only the latter two are eligible for mileage upgrades.

Nowadays, sometimes M is considered "Econ Essential", and sometimes it's considered "Econ Light". One can easily be fooled into thinking they are eligible for a mileage upgrade when buying M-class when in reality they are being booked into "Econ Light" which isn't eligible. So my take is instead of being fixated on what the exact fare-class is, perhaps it might be better to focus on whether you're booking Light, Essential or Flex instead.

This change in fare class presentation took me a while to get adjusted at first, but I can't quite say it's a bad thing now. These names seem easier to remember than the fare class codes if you aren't familiar with them.

Same concept applies to Y+ and J.