In theory, the planes can fly with full pax+bags+cargo the entire way, but if the aircraft itself isn't officially certified to carry the extra weight, the flight becomes weight restricted, as we've all seen from Day 1. FCYTravis touched up a little on it above. If HP wanted to, they could pay extra to get the paperwork updated and the aircraft recertified towards a higher max weight. We see UA with nonstop DEN-KOA and CO's 752 Transatlantic services because those aircraft are most likely certified to the max gross weight as designed by Boeing (~255K). HP's 752s, IIRC, are certified up to 210K.