Hello if I end up flying JAL instead of ANA. Can I get boarding pass airside when I land in Narita? I heard they fly out of different terminal?

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In principle yes, as long as you can document your itinerary to United when checking in, and leave time enough between flights to take the inter terminal airside bus, you can get your JAL boarding pass in T2.
But you will not be able to check luggage.

I have an overnight HND transit. Arriving at 2pm (UA), and departing 11am next day (NH) on premium econ, single ticket. I'm *G and have Priority Pass. Will I be able to access the ANA lounge on the day of arrival?
I understand *A lounges are based on same-day departure or before 5am next day, but unsure if I would be able to get in using my priority pass.

If the lounge is not too crowded to accept priority pass then it should be fine. There may be time limit though?

Thanks. No time limit stated in conditions on priority pass page.

I know I've seen this somewhere, but not sure where.

Any experiences with arriving at HND, having a 5-hour layover to get to NRT, and catching the next leg of a flight? What method does one use to transit between HND and NRT? It seems really silly, but there's an award ticket at a killer price and I might be willing to suck it up.

I've done that pre-Covid. There are/were shuttle buses but I just booked a Blacklane driver. Met me on arrival at 5am and I was at Narita less than 40 minutes later. As I recall it was roughly $200 back when the yen was much stronger.

There is a limo bus that brings you from HND to NRT that runs frequently, ~80mins travel time and costs 3200 Yen. https://webservice.limousinebus.co.jp/web/en/Top.aspx Click on "Narita airport" to make a connection to NRT and select Haneda airport as departure location in the next screen.

Wonderful, thank you so much!

A hired car or bus as mentioned, you can also take a train which will require at least one train, or be on a standard Japanese commuter train layout with no real space for luggage for the duration.

I'd probably rank bus as the best option, just check the schedule in advance during covid it has been quite spotty, and I don't know how quick they are to ramp up post October 11th.

The Blacklane price mentioned is certainly very attractive as I guess that would have calculated to about 25K yen, and they mostly charged 15K Haneda to the city pre pandemic. But if you'd like to splurge a bit, and with the yen what it is at the moment, it could be an attractive proposition, check with Tokyo MK as well, they could have some great prices as well.

Is it possible to connect from NRT to HND with an Evisa and US passport?

Not sure what your question is? If you're legally allowed to enter Japan, you can go anywhere including doing this transit.

Fly into NRT and out of HND the following day.

An Evisa implies you can enter Japan, so nothing would stop you. The problem until recently was to get a visa. From Oct 11, visa waiver will be reinstated and you don't need it.

It'll be about $150 to $200 at current rates plus 1 to 2 weeks wait, so it makes for an expensive transition.