Whoah... !!!
Here is what I believe is the plan. Random booking YUL-EZE in January (21-29) 2014. Tango return is 1057$ which is probably 3-400$ less than what it used to be, at least 2 years ago. Flex return is more or less 1600$. So if you're below SE level, and you want to upgrade, you'll need 34 credits + 1000$. Your J seat cost you 2600$.
Now look at Z class price tag: 3472$ return...
They lower their price for the mass market leisure/cheap traveler, they try to push corporate spending up by bringing Y folks in J at a lower price, and they abandon all the mid-level frequent flyers...