If one is on a reward flight (business flex) and at the time of booking (even currently or just prior to check-in) the first class flex cash price is lower than the business flex cash price, will Swiss charge an arm and a leg (like 1300 CHF) to upgrade from biz to first at check-in?

Or will it be something nominal (like 100-200 CHF) to show they got some money for a bid?

How does that work when flex cash J is more expensive than flex cash F and how does it affect a non-M&M reward booking (since normally they are not allowed to upgrade on their own program's points)?