Quoting Darrenm88
Like if I extend my New zealand flight, it will make my time in the next stop shorter?
Yep, unless you change the departure date from that next stop as well (which will cost you another 30 quid per change). You should probably also be aware that that date change fee might very well be different for some of the other stops! Also, if there is no availability in the airline class you are flying a week afterwards, and you really have to fly that day, then you'd also have to pay for an upgrade in class I'm pretty sure. That's typically not cheap.
One thing I should point out though is that most backpackers end up making at least one date change after they've left home. So I wouldn't be too worried about getting all the dates 100% in order just yet, as chances are you'll want to change them again at some stage anyway depending on people you meet, routes you choose etc. 