its not an easy task because the jamb is usualy fixed to the head and cill if it has 1 by mortice and tennon joint and screws so you might have to take the full frame out to replace the leg and you could damage it by taking it out,it would be easier if you replaced the full door frame then re-hang door.hope this helps
shaun www.sdwupvcrepair.co.uk
May 2008