Had no luck for many many times with many many phone calls, except once:

UA cancelled one segment, ANA couldn't arrange an alternative solution without a forced overnight stay, so I had to take care of that segment (SFO to US local) by myself which I ended up purchasing a separate Alaska airlines ticket. I politely asked, in turn, can I change other routing without cancelling whole reservation? I wanted to change routing but some segments weren't available anymore and I couldn't risk of losing them. After a long pause, ANA let it happen as an exception. It was a RTW award ticket.

Except that one case, ANA has been very strict on the rule. Never ever let me change routing without cancelling it.