Of course you can upgrade a complete portion at one time. You just have to wait until the last leg's departure is inside your upgrade window and then you can request upgrades on all of them all at once. Of course, you are taking the chance that by waiting an extra few hours, your first legs will get snapped up. Snoozin' = losin'.

Look, if I book a YVR-LHR non-stop for 18:15 departure, I can only u/g 4x24 ahead of 18:15 PT on a T+ fare. But if I routed it YVR-YYC-LHR instead, one of the eligible bookings has me leave YVR a full 7 hours before the YYC-LHR leg departs. There is no reason whatsoever that just because I did a 2-segment flight instead of non-stop, that I should get to upgrade the long leg 7 hours before any YYC-originating pax can.

That's an old way of gaming the system in which a YVR-originating pax can screw a YYC-originating pax out of a fair chance at an upgrade. It sucked when it was permissible, and any fair-minded person should be happy that this is no longer possible.