If there are seats available for upgrade purchase at the time you check in, whether you decide to OLCI, or do so at an airport kiosk, it should bring up a screen with the option. If theres no NF availability at that time, it will likely ask If you want to buy up to E+, if available. If you OLCI and it doesn't ask you about an F buyup, you can always check when you get to the airport as there could be inventory they decide to open up by then.