Best to phone your local SkyWards office and explain what you want to do, may be they can check whether there are seats available before you purchase the extra miles.

I called the other day to upgrade on one sector of my next flight BHX-DXB-BKK RTN, I wanted to upgrade on my last sector [DXB-BHX] so I have a limo to get me home, upgrade & limo booked and confirmed in one phone call.

BB