PMUA overbooked business class at times, but in PMUA you could not have business class without a third cabin known as first class. So in an oversold situation, the recourse was to bump a business class pax to first.

The merger introduced the abomination of two cabin long haul.

Also, I cannot recall if P fares predate the merger (I believe they do) but post merger P fares:
- are apparently regarded by UA as an economy fare that books into the next class. Thus UA tries to weasel out of compensation.
- much more expensive and priced where Z used to be priced. Z is now priced where D used to be priced. This is intended to limit the use of GPUs


So I think we can hang all this on Houston.