I don't know about KE always being booked full up front. I've been on a bunch of their intra-Asian routes, and it varies widely. Sometimes fully booked in Business, sometimes not. First Class is rarely full, and often has just one or two pax. Don't know about their USA routes. I've asked about upgrading economy class tix in the past, but the problem I had was that mine were DL codeshare tix, meaning I had to rebook to Business Class with DL.