Well is your system R134 or i believe R12? If its the R134 just get the gauge and freon at advance and do it yourself. If its the R12/(4) you have to first get it discharged by a professional then get the AC retrofit kit from Advance/Autozone and go that route.
http://www.id-usa.com/catalog.asp?CID=28
That site there will show you videos on how to do it. It is not that hard to do just make sure you do it right and you wont have problems.