I, too, have an upcoming reward flight that leaves in the middle of the night. And for the same reasons as you, I'd prefer a day time departure. Regardless, I'd always take F at night to C during the day. Remember that the flight is 14 hours long or so, so you won't be sleeping the entire flight. And Cathay will serve you whatever food you want at any time. Cathay F is truly aspirational travel. There's no comparison between F and C in terms of the personal comfort and space that you get upfront.

Whose miles are you using? If you are using AA miles, once booked, you can change to a different flight without a fee change as long as your originating and your final airports do not change. So if you have SFO-YVR-HKG, you can change to SFO-HKG fee free if it becomes available. Cathay availability isn't predictable. (I have no idea if AA flies SFO-YVR; I'm simply using it as an example.) I'd grab the best of what you can get right now and continue to monitor for your more desired flight.