In my experience it all depends with the agent at the counter. On average though, if your original fare class is available for the standby flight the counter will do it. If there's only a few seats left (this appears in your case with only Y4), they'll make you sweat it out at the gate. It helps that you are an elite. But just keep checking the class availability (flyaow) b/c I imagine more seats will be released as Friday approaches.