If that’s the case, the solution would be to book each leg separately as a one-way trip. In theory, UA can do this after the original ticket is issued, but I’d probably look for a way to do it at the original time of ticketing. Your problem, if any, is going to be documentation — for auditing / tracking purposes, it’s possible that you’re required to keep all travel for a given purpose within a single record. That’s the question you’d want to ask, though — are you allowed to book, e.g., GUM-HNL, HNL-(NRT)-OKA, and OKA-(NRT)-GUM separately if the price is the same. This would also solve your upgrade re-pricing problem.