You'll need everything essentially. The only thing you can re-use is the axle housing and the gears. You'll need new shafts, bearings/seals, carrier (and everything associated with the carrier).
I guess you could machine your carrier to allow the 29 shafts to fit, but for the $$ in doing that you might as well buy the correct one.
It's honestly probably cheaper to just find a 29 spline axle than to upgrade unless your going with aftermarket shafts/gears/etc anyways while your doing it.