So I'd like to return to GRR instead, which I'm guessing has similar rules.
I see there is confirmable PZ space too.

I'm booked in (N) (which is actually ANA premium economy, despite UA showing regular economoy).

So Here's two additional questions:
When I ask for this change, will it matter if I ask for the NH native or UA codeshare number? Also, The fare class I had (N), is not available, but G and E are. When paired with a connecting UA number, UA gives the Codeshare number with different fare buckets, and I'm unsure of fare mapping (but could look it up). I'd like to get E or G so I can use a GPU, that would be my goal, I just know this is all going to confuse whatever poor UA agent picks up my call.

So I'd like to switch to BKK-NRT-IAD-ORD-GRR, or if that iad-orr is a violation, BKK-NRT-ORD-GRR (iad-grr works but is a crj200, no polaris lounge). But I'm guessing the R fares might have a provision that one of the flights has to be on a flight that has R? Otherwise, why would the fare exist? Not sure if this is related: