Regarding Business, they are correct. (Regarding PremEco, I don't know.) Before that change mid last year, U was called K, which might explain some of the confusion.
U is the cheapest upgradeable fare if you want to fly Business.
Alternatively, depending on where you fly and when, buying up to S class (the cheapest upgradeable PremEco fare) might not be much more expensive than buying up to U. Overall this might save you money as you will need only half of the miles for an upgrade. (And you will earn more miles on the flight, too.) But this really depends on the particularities of your ticket. (Check out S fare for your booking and flight dates on expertflyer.)