This depends on flexibility of your ticket. If it allows changes, you can usually change the routing subject to any fare difference.
Given that OSL and IST are quite a bit apart, the fare difference could cost you a lot as it is possible this would no longer be considered a return ticket (I'm not sure about LH's policies). It would be better to fly somewhere in Europe. Have you considered ARN/GOT? Sweden currently accepts travellers from New Zealand.
For more details, call LH, this kind of change usually can't be done online.