I think you may be confusing things here. By "long haul flights" you seem to be referring to international flights of the sort one would buy a W fare (or higher) to upgrade with a GPU. Those flights are not eligible for complimentary premier upgrades. So to answer your specific question, no, a B fare will not upgrade on those flights without an instrument (GPU or miles+co-pay). Now between two pax who are trying to upgrade with a GPU, one on a W and one a B, the pax on the B fare will get upgraded first.

Edited to add: Unless the pax with the W was a GS vs. a 1K on the B, in which case the GS trumps the B fare.