This was a domestic flight, so the international prohibition on movement between cabins did not apply. Nonetheless, the correct approach is for the passenger seated in F to ask the FA if the swap is OK and carefully note that it won't be until after the meal service and when the aisles are clear. You won't be denied absent some issue such as turbulence.
Same thing goes for international Even though it is theoretically prohibited, the FA's know if there are air marshals onboard and won't care if you are decent about it.