If Y is wide open when you get to the airport, often the ticket agent will process you into Y instead of SBY when you check your bag. At that point you should be on the F standby list for battlefield upgrades. I've also had ticket agents just hand me an F ticket when I've arrived early and asked to go SBY on an earlier flight, but that was back when I'd be on flights that actually had empty F seats at wheels up.

About 4-5 years ago I was next on the list for SBY on a flight, Y was full, F had one empty, and they'd already processed every eligible upgrade of confirmed pax. The two G/As were discussing their options when I pointed out that if they put me on the flight they would have somebody they could upgrade.