As Zirconmermaid said,
heat! With Obitsus, your hair dryer is your best friend.
Most vinyl parts are held in place by a joint at each end. The hands, head, and feet are the exceptions. The head has a special connector to keep it in place (most of the time). The hands are excruciatingly tight on the wrist joint (I've snapped a few wrist joints removing hands). The feet have a screw in the base, which doesn't show most of the time. In flat feet the screw also holds a small metal clip that keeps the foot magnets in place. In the Parabox tutorial, they show disassembly of the the entire lower leg. I think they are doing that assuming your doll has the older, taller ankle joints that came on early 50cm bodies. I'm pretty certain all new "50cm class" bodies (40/45/46/new 47/48/50cms) come with the shorter ankle joint. Since you have a 48cm and a 40cm you should be able to leave the ankle joint attached to the leg. Just remove the screw and heat up the foot (if needed) and slip it off.
Old taller ankle part:
http://www.obitsushop.com/SHOP/50RP-F01WP-29.htmlNew shorter ankle part:
http://www.obitsushop.com/SHOP/50RP-F01WP-35.htmlNote the difference in height of the stem under the joint ball.