So let me get this straight.
For starters, a clear definition of terms. There are basically three boxes I am dealing with here:
1) a central junction box, from which the cables to the old switch and the new switch go.
2) the old dual switch box. This is where I am trying to replace a 2-way switch with a 3-way switch. There is 1 3-conductor + ground cable going to this box from the junction box mentioned in step 1. As i mentioned, that same cable is also used for the other switch in the box as well as the light it operates.
3) a new switch box. This will have other 3-way switch and will have a 3-conductor wire (plus ground) from the junction box mentioned in step 1.
So what I didn't understand in your reply JoeD was from which box do I need to get a 3-way conductor cable. From the dual switch box or from the junction box?