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.