I don’t have personal experince to give you but I would guess it would be easier to book the extra trip in your original booking with dummy December dates, then later move them back to July. That way you will only be making a date change, not a routing change. You also need a clear answer from BA on ticket validity (a one year window): your initial ticket will be valid for 1yr from booking (Dec17-Dec18); you’ll need it reissued in July for the later flights (becoming Jul17-Jul18) which should be straightforward but I’d check when you book.

Do consider that you might be able to avoid fuel surcharges on your whole booking if it starts in HK: it might be better value to make a normal offpeak BA one way to HK, then start 25k award from there?