If you are going to replace the o-rings in the system... It has to be discharged completely first. Then all the lines can be opened and the seals replaced everywhere.
If you are just going to try the re-charge then you don't need to do anything else... The dealer will add refrigerant as necessary.
It might be fine aftef a re-charge if the o-rings didn't dry out or crack. If you have some kind-of-bad seals then it might stay charged for weeks, months, maybe even a year or two... Depends on how bad the leak is. If the leak persists the "no cold air" condition will return.
When it's at the dealer... If the a/c machine detects a leak the dealer will tell you and likely try to charge an arm and a leg to diagnose and replace whatever seal is bad. The process usually involves injecting a dye into the a/c lines and then looking at all the system lines under u/v light to see where the dye is leaking out.
It is possible to have taken a rock-strike to the a/c condenser that sits in front of the radiator... Could cause a leak.... But with an older system that hasn't been used regularly, I still suspect a seal somewhere.
Another possibility is a bad pressure sensor in the a/c system. It tells the computer whether or not there is sufficient charge on the low-pressure side of the system for the compressor to turn on / engage. If that sensor is bad, or unplugged, the a/c compressor will never engage. There is also a electrical connector on the a/c compressor that engages the electro magnetic clutch... If it's unplugged the compressor will never engage. I only mention these because there are stories about rodents chewing on the wires of cars kept in storage... So if you keep your car stored for long periods of time, where there could be rodents... I thought I'd mention it. You'd likely have other electrical problems too though, as the rodent(s) munch on other wires.
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