I have a J class ticket booked during last NOV/DEC's promo. Routing is yyc-lax-icn-nrt nrt-ist-lhr lhr-fra-yyz-yyc

From my readings I think I can make changes after we depart from Calgary and we will not be subject to repricing of taxes and points, is this correct? So I will just have to pay the $90 change fee per person?

What should be my strategy for changing to an AC flight from LHR-Canada ? Day of or prior depending on loads?

Thank you