Yesterday, I had to change the return leg (LAX-IAH) of a Y class reservation leaving today and returning on Friday. At that time, there were no Y-up spaces available, so I was assigned a seat in back. In reviewing my reservation today on CO.com, I had not been automatically upgraded (although the auto upgrade option was shown as "On"), but a Y-up upgrade was shown as available. I was able to select the Y-up upgrade option and I was moved into First (yea).

Should this upgrade have been automatically processed or do I have to remember to check the web site to try grab a Y-up upgrade before the 24-hr battlefield window?

P.S. I'm Plat elite, so I think that I would have gotten the upgrade anyway under the battlefield hierarchy.

You would likely have been upgraded when the EUA process ran, assuming there were no other elites with a higher priority (i.e., higher status or earlier booking). IIRC, the EUA process runs around the same time of the flight, every day for 5 days prior to the flight departure (i.e., at 24 hours prior, 48, 72, etc.), and then it runs again 3 hours prior to departure.

However, I wouldn't rely on this, because someone purchasing a new ticket could easily snag the seat. This is where ExpertFlyer.com would come in handy - it can proactively alert you of the availability of the "A" inventory you'd need to upgrade the Y fare. Or you can just keep checking periodically.

Y-up comes out of A inventory; EUA comes out of F inventory. The Y-up won't happen automagically if A inventory suddenly shows up so you have to stay on top of it yourself.

You probably would've gotten it as a plat in Y when the gate started processing it, but that assumes that a seat would be available, something that doesn't happen often on the CO transcons.