Based on my experience the worst route for upgrades, at 7 days out, is FRA. I hear HKG is difficult as well as SYD.

I have never had any trouble with LHR, but it has been risky, but has always cleared at standby desk.

MUC is generally OK.

FRA is difficult out of YYZ, but what about YUL?

And of course YVR-YYZ (& YOW)is always risky!