Exactly, especially because UA loaded the widebodies but took at least 2 weeks to exclude them from most of the cheap UPDI fares. On the 744 SFO-ORD, you could purchase a P for ~$400 (same as the regular narrowbody) and A (Booking into first) could be had for ~$700. By the time it was corrected, the cheapest J was in Z for ~$800 and F was ~$1-2k (but the F cabin filled up quickly).

Using today as an example, Kacee's estimate was pretty spot on. 744 SFO-ORD, I count 35 Y>J upgrades and 9 J>F upgrades. Which means F was 75% upgraders (9/12) and J was 67% (35/52). However, before the J>F upgrades, it would've been 50% paid 50% upgrade (35-9).