Yes. KA 3-class plane does allow you to assign F seats closer to departure as cxfan1960 said. I was on full J-fare ticket to a mainland Chinese city, and originally I was only able to online pick the C class seats. However, closer to departure date I was just double-checking my seat (a habit ever since Cathay changed my seat due to aircraft change, to a really bad one, that I didn't check), and I was surprise I can click left-arrow, which are rows 1 and 2. Picked 1A.

No big deal really since KA "First" is Cathay long-haul C seats, but it's nice to be able to lie-down and rest.