I did TS2 to TS3 body conversions for my Servo and Skeleto sims, but they were 'easier' because their bodies are just 'sticks' and don't require muscles or morphs, and even that was tedious enough that I'm (probably) never going to do it again - but reassigning all the joints for regular clothing plus repositioning the TS2 mesh arms to the position needed for TS3 arms, and then having to make morphs, is a whole new world of pain (I've tried!) and as Bloomsbase says, you don't want to go there!
The best thing to do is to find a TS3 mesh that's as near as possible to what you want to make, and reshape the mesh using the TS2 mesh as a guide. Then use the TS2 textures as a reference pic for your TS3 textures (or if the TS2 textures are high res enough, you can painstakingly copy and paste each body part onto a TS3 texture (you have to do it part by part, because the location of each of the body parts on the texture map is different in TS2 from TS3). Still quite a bit of work, but a lot easier than doing an actual conversion!