Do EU downgrade regs apply for PE to Econ, or is there some way by which they're both considered economy (since traditionally there's only been F, J and Y)?

I would think that if your ticket was clearly ticketed into a Premium Economy class, and you had the impression at the time of booking that would be honoured (I don't know how carefully you looked in advance, but you say you do this flight routinely), then getting to the airport and being told you're in "common economy" could be a sort of involuntary downgrade. A penalty could be due in such a case... even if no penalty, a fare difference to the "common economy" fare could be due.