So I didn't think to document this process WHILE I was doing it but I can explain fairly well. It's a simple procedure but I believe DollyKim asked how it was done? I think? I meant to post them a while ago, but forgot. To do this, you'll need a very sharp craft knife, with a point narrow enough to get into the legs, and a piece of elastic of the same size and tension as the sort you'd use in a strung MSD, about 6" inches long. I get mine by the yard from Jo Ann's. It should be big enough for you to thread through the legs and crotch and be able to tie a knot in, then trim it.
This is after the swap. You can see the barbie in pieces at the top has no joint in her torso. She was one of the Think Pink dolls; Barbie's hispanic friend Teresa. From 1995 I think. The separate legs were originally part of the doll that is now whole. I think the doll was Perfect Pink Teresa. Came with a brown shirt, pink overalls and tube of temporary hair color.
So first, here's a picture of the damage to one of the feet. These little tooth marks go all the way up both legs to the knees, and there's a nibble on one thigh as well. Not really fixable, the gouges are pretty deep. Besides, I don't like the flat feet.
I stupidly thought I'd have to break open the crotch in order to get the good set of legs out without breaking them, but I realized halfway through the process that the black rubber is easy to cut.
This is the groove that you need to be able to fit your knife into. There should be a chunk of rubber in there, holding the legs together through the groin. Wiggle your knife in there, and slice the rubber. You'll have to do this on both sets of legs. And of course, when messing with knives, exercise caution. Cut the rubber free of those pegs and toss it. It's useless now. Please excuse the quality of the next few pics. I forgot to turn on the macro setting on my camera.
Now take your elastic string and thread it through the peg of one leg. Put both ends of the elastic through the crotch, and then thread one end through the peg of the other leg. Pull the elastic as tight as you possibly can, and tie it off securely. Trim the excess string.
Wiggle the knot into the body again and you're done.