happened to me for the first time last week as well. They called on Friday to offer upgrade to C on the flight to NBO on monday. CHF 760 (or so) - I took it...
Original booking class was B - new one was D
Surprised to see they offer it for "cheap" classes too though!

(this is actually quite nice, because I don't get into trouble with the company as I have a original receipt that states Economy... now I just have to put the 760 onto expenses with a creative reason....)