If booked as GUM-HNL in one segment with 5 stops, you can only choose a single seat for the whole journey. However, at T-24h, during check-in, you should be able to change seats on each separate leg.
I succeeded in asking an agent to split GUM-HNL into 6 different segments, but YMMV - most people were unsuccessful.
Thanks.
You called and split the leg into six segments way ahead or at T-24? I am elite, so no additional cost for E+, besides I believe this may actually enhance my chances at upgrade as well.