Ive only changed terminals at NRT twice. Connecting from Aeromexico arriving in terminal 1 to JAL in terminal 2. IIRC it was a one way transfer and one could not just freely board the bus back and forth without a flight in the other terminal. But I also recall neither time was I checked in for the JAL flights prior to arriving at the transfer check-in counter in Terminal 2 so I did not have my boarding pass prior to boarding the bus in terminal 1. I do not recall what, if anything, I actually had to show to get on the bus to change terminals.

This is relevant now because coming up I have a 4 hour JAL to JAL layover all in terminal 2 but I have access to either the Delta Skyclub or Korean Air lounge in terminal 1, IF.... I can make it over there. I get no love at all in terminal 2. I do not think they will let me on the bus just to get to a lounge but what do they actually verify before they do let you on the bus??? Could I just verbally say I need to check-in for some flight at the transfer desk in terminal 1 and get on the bus that way?