GPU usage on continuing flights

I found (the hard way) the other day that if you are on a flight with continuing service and aircraft change (EWR-HKR-SIN on UA 117 in my case) that you won't be upgraded unless seats are available on both segments.

The gate agent immediately knew what this was about when I asked why my position went from first to non-existent. She claimed that upgrading me would result in overbooking for business seats on the second segment!