Has anybody been successful in changing the routing of the return leg of an AP ticket, after departure?

I called, and as per the rules, the agent [quite rightly] informed me that I could only change the date, but the route must be identical.

I'm currently scheduled to fly back from Paris in LH J, but a seat on NZ's LHR/LAX flight just opened up and I'd love to try them (plus, it leaves much later in the day).

Any insight?