First, see this link (
here). Call again and see if they will address your issue, or allow you to switch to TG without reticketing everything.
Realize that once a round-the-world award is ticketed, the only permitted change is to the date of travel. The airline, route, or cabin cannot be changed without re-issuing the entire ticket, which would require that all of your flights still have award availability. ANA charges 1,000 miles to make any date changes.
Therefore, if you must change to TG, and ANA cant do anything to help you, then begin by making sure that availability exists for not only the TG flight, but also for all other flights in your itinerary to begin with.
Next, confirm that you have fewer than 8 stopovers in your new itinerary. (A stopover is defined as any stop greater than 24 hours. Your change could trigger the 9th stopover)
Call ANA again and have ready the TG flight number, date you want ready to change it.
Expect the usual change fees and conditions for RTW bookings.