With paid upgrades there is no longer the predictability as in the past. You can be a high spend EXP or even CK and miss an upgrade on a CR900/E175. You can be a lowly GLD and get upgraded on a DFW/LAX type of flight. It just depends on how many people on the flight will bite for a cash upgrade and to a lesser extend deadheading pilots, the latter more seen on trunk routes. And since that has very little predictability you should assume that if you really want to be upfront pay to be there. Otherwise, prepare for steerage but hope for the best, an upgrade, preferably ahead of the gate.