I have done similar changes pre-covid, I am trying to do the same for a BOS-TYO-HKG flight now. No J availability when i search BOS-HKG directly, but when i search BOS-TYO, and TYO-HKG separately, i see awards available.

I HUCA'd twice and, all three agents told me that I cannot join those two legs, and one of them suggested to book via the multi-city option, which essentially book into two awards for the trip.

I am curious what did you do to make it happen. Did you put one of the segments on hold when you called? Or did you ticket the two segments already?