You already explained it in two words.
good point!
You will be able to upgrade to C+, but ONLY on day of departure. Most of your other benefits will carry over. You'll get lounge access, sky priority baggage/security etc.
Thanks, I was afraid of that. I really want to book c+, since it is a 767 I am flying on. Doubt that they will have any good seats left on day of departure.