I just booked a cheapo $560 K ticket from YYZ to SIN via SFO next Jan as a mileage run for TK.
Upon booking, it gave me the option to upgrade SIN-SFO to Premium Plus (proper PE on UA I assume?)for $328...I took it.
I did notice that it said mileage accrual based on original fare on the confirmation page - does that apply to UAMP only?
When I go to manage my booking, it is now showing proper R for the segment instead of K. If I can get 100% instead of 50% on TK for this, that'd be a steal.

Many thanks - I flew 60-80k miles a year on UA pre-pandemic, but I have been crediting miles to TK since i travel for leisure to exotic places primarily and TK serves me well in that regard...occasional long haul business travel I get to book lowest J but ST is usually cheaper than *A.