Originally Posted by
Thijs Luyt
They are only allowed and able to offer upgrades to La Premiere for Platinum members that purchased a J fare ticket and he also said that not all J fare classes are eligible for an upgrade offer. Deeply discounted J fares are not. Even though a Gold (or less) FB member had a full fare J ticket, they will not offer an upgrade with money or miles.
I was offered a paid upgrade from a low J fare (Z) to La P a couple of weeks ago. My wife (Silver) also received the offer. The price was far too high to be tempting.