But for such a long trip it's worth spending some time in planning the flying destinations as you might want to do some interesting flights (which you didn't think of originally) ... and not do others that you though you would !
For instance, I didn't do the flights in South America I was planning as I went on land. On another had, I would like to stop on Easter Island and Tahiti, on the way to New Zealand !
I could have changed my tickets for a fee but that wasn't possible as the flights were full.