Some may tell you that the pump can be removed by isolating the service valves on each side of the pump,thus only losing a small amount of refrigerant,can be done but not a good idea especially if its r12 (unlikely, but it can result in a $10,000 fine in this country)The oil in an air con system does not stay only in the compressor it will migrate around your entire system.Point being that if your oil is contaminated from your present unit you will cary this over to your new pump. Have a fridgy reclaim the gas and re charge with new ,this will ensure that you get the correct charge in the unit as it will be measured from empty.May be a good time to move from r12 if thats what you have at pesent . regards HOP