AC systems are hermetically sealed systems. They operate with refrigerant and special refrigerant oil. Moisture is bad for AC systems. Whenever a system is charged, it is evacuated to a certain vacuum and then filled with refrigerant (and oil, if necessary). Moisture combines with the oil under high temperatures and pressure to produce acids which attack your AC system. Just in case the slightest amount of air (or moisture) got in to the system during a repair, or the charging process, there is a dryer in the system which chemically absorbs this minor amount of moisture.

When you break the hermetic seal of a system, if moisture gets in to the system, the dryer will absorb that moisture and the chemicals in that dryer will, eventually, be "used up." So if your AC system has been sitting in a non-hermetic state for more than a few days, it is possible that the dryer has been "used up." Whether, or not, your dryer has been "used up" is dependent on so many intangibles that it is common practice to replace the dryer whenever you are doing a major AC component replacement.

So if your car has been in a non-hermetic state for a while and you do this condensor change-out yourself, replace the receiver/dryer, close up the system, and go to your local AC shop to have the system evacuated and recharged. Preferably, do this whole process within 24 hours of your repair so that your new dryer isn't "used-up" by the time you have the system evacuated.

BTW--you cannot "recharge" an AC system with the little cans you buy at the discount stores because you have not evacuated the system. So, if you try to use those little cans, your system will be dealing with a mixture of refrigerant, air, original oil, the oil they supply in the cans, moisture, etc. It may all appear to work, but it wasn't designed to work like that in the long haul.