I agree with lucky. I wouldn't book a Z fare on UA for a Z fare because of the chance of a last minute substitution to the old equipment, and in that case Z won't get you a flat bed seat.

If it's a C or D fare and it isn't heavily discounted -- sucker! Whoops, slight editorial comment sneaked in, apologies. Anyway, I would still upgrade because of the relatively high chance of an equipment substitution.