Booking partner reward flights (complicated error)

Im currently in the middle of nowhere in Laos. I flew VS from LHR-NRT then from NRT-BKK with ANA (NH). I will be going back to Bangkok to fly to Tokyo on the 26th of August.
I also have booked an ANA domestic flight from Tokyo-Osaka 28th-30th August. The problem is, this should be till the 31st. The ticket was booked with VS ff miles but ANA is the carrier. According to the ANA ppl over at flyertalk, I can change the ticket 4 days beforfe the departure and I have to go to the ANA ticket office in BKK (obviously there is no VS one here). But someone else said VS will have to change the ticket. Does anyone know for sure how I can change this ticket and how much it will cost?
Many thanks
I also have booked an ANA domestic flight from Tokyo-Osaka 28th-30th August. The problem is, this should be till the 31st. The ticket was booked with VS ff miles but ANA is the carrier. According to the ANA ppl over at flyertalk, I can change the ticket 4 days beforfe the departure and I have to go to the ANA ticket office in BKK (obviously there is no VS one here). But someone else said VS will have to change the ticket. Does anyone know for sure how I can change this ticket and how much it will cost?
Many thanks