Originally Posted by RobOnLI

socrates - NW and CO do it basically the same, except CO's program is more flexible. Here's what I mean:

On NW - Y26 and Y27 fares guarantee you an immediate upgrade if the flight has "P" space available. "P" being the main FC bucket, of course, which means so long as the airline was willing to sell a seat in FC on the flight and an elite purchased a Y26 or Y27 fare, the elite would be upgraded automatically at time of ticketing. No waiting for 5/3/1 EUA windows.

NW also sells some tickets that provide for immediate upgrades provided space is available. These are usually shown as M2R, H2R, etc. in the fare class. You are technically buying an "M" or "H" fare but you are booked into "R" class which is also first class. In my experience, you will only see these fares if "R" space is available on the flight when you are attempting to purchase a seat on it. Further, these fare classes are not specific to elites and are open to the general public.

On CO - all Y and B fares (regardless of the "sub" class, if you will) allow for elites to upgrade at the time of ticketing if seats exist in the "F" bucket. Technically you are upgraded to "CU" class but it seems that CU is equivalent to "F". These upgrades are available at time of ticketing for all elites on CO & NW.

On DL - it appears that even if you purchase a "Y" fare which allows for an "immediate" upgrade, you are only upgraded if availability exists in the upgrade buckets (X or V). This is totally different from NW and CO who pull straight from their main revenue first class fare buckets for these upgrades.

-RM