The refrigerant in the system needs to be recovered using proper refrigerant recovery equipment.
*Warning, venting refrigerants into the atmosphere from most existing AC systems is illegal*
Chances are that your old system contains R22 refrigerant. A new residential system will use R410-a. The two refrigerants are incompatible and the lineset should be changed. Let me know if you want me to list the reasons why.
"The physician can bury his mistakes, but the architect can only advise his clients to plant vines." -- Frank Lloyd Wright