If I purchase an M fare LAX-SYD and there are seats available when if ever will DL allow me to upgrade using SWU/miles?? I am PM and don't want to make the trip in coach but is buying the M fare is a gamble??

Think I read about day of departure YBM upgrades, when does that start??

I feel like I should know how this works but I have always been able to PMU/SWU's when I need them.

Thanks.