Question[s] for experts [Mods, please move if this is the wrong place, I tried looking elsewhere].

I'm booked on a shorthaul flight LHR-EU that is showing as zeroed out in all classes but J1, and I'd like to volunteer to move to the next one [currently showing better availability] and help BA if they need extra space.

I have 2 questions:

- am I better off doing OLCI or not [will arrive at the airport several hours before the flight, so will be among the first one to check in there]
- if I do OLCI, am I better asking about this at the F check in desk or in the lounge/at the gate?

I won't be checking any luggage.