You will lose the 15% online booking discount if it was booked online; otherwies, IIRC, there is no fee for changing the flight times.
Even if these were SQ tickets, wouldn't there be some admin fee on top of the revokation of 15% online booking discount? I would imagine that someone who redeems the ticket by phone and thus not eligible for the 15% discount in the first place wouldn't have the luxury of making unlimited changes. Perhaps that's allowed for Flexi tickets but not Saver ones?