Well I thought I would give it a shot and called QF (US).

After some confusion on the part of the CSA, she realized all I wanted to do was to change the dates of sectors (she was worried i would move outside the rules - this didnt apply as I would be flying same sectors).

Anyway she happily changed the dates on my sectors so that LHR-DXB-LHR now comes before LHR-RIX-LHR.