Switching SIM card

Question

I have Cingular wire service and got a free phone when I added another line to the family service (Motorola something or other and I can't hear on it and just hate it) I bought a Motorola V170 Tracfone ($20) and want to use my Cingular SIM card in it. How do I switch? I've been told it can be done, but I haven't found a solution yet. I can't afford to buy another phone thru Cingular.