I definitely thought about married segment, but that "shouldn't" apply as I had a several month stopover. This is the final segment on a 5 segment itinerary.

I can't find on Expertflyer any way to check POS (for fares, yes but not for flight availability), however I just did a simulation of my original itenary as if I didn't have the stopover, and actually the connection I want onto JL 33 is I9 that day as opposed to I3 or I4 when just checking from HND-BKK. That certainly makes this even more interesting. So there's definitely something preventing me from making the change HND-BKK on JL 33 but allowing NRT-BKK on JL 707 (same day), if I were going all the way JFK-HND-BKK without the stopover the I class is absolutely wide open. Head scratcher.

I also double checked the current fare rules on Expertflyer (my exact fare code isn't on there anymore, but all the current round trip I fares have the same text), and there is no flight limitation preventing use of JL 33. They just simply say any JL flight operated by JL.