I just finished a *A RTW tour and had no problem changing both the route and dates of some of my flights. Ticket was issued through LH and when I called it was a very simple process to get the new flights booked (I did my homework checking availability) and after a few hours received the new itinerary via e-mail.

And it wasn't just for LH flights but also NZ and OZ. No problems checking in either, even reserving seats just after receiving the e-mail conformation.

So it can be done :-).