However, both Y and B classes are "full fare" categories and in some airlines booking into these buckets makes upgrades very easy, if not complementary. I would certainly think that at an airline's discretion you could be put into a PE seat if one's available. I've had gate agents and counter people waive formalities when confronted with a DONEx ticket - even bumped to first class when J was overbooked - so I wouldn't put your chances at zero, far from it. They're not robots after all.