From a diyer - you will lose whatever freon is in there AND it will make a big mess. It can be done - when you remove the switch just make sure you have some goggles/safety glasses on, slowly unscrew the switch and do not stand in the direct "line of fire" while removing it. In addition, the longer you leave the system open the more time that moisture has to get into the system.
I would suggest finding a good independent a/c technician, taking the vehicle to him/her with the new switch and having them do the job.

I usually take it to my a/c technician, ask them to remove the freon, do whatever work is necessary and and then take it back to them to run a vacuum and recharge the system. Figure at least an hour for them to run the vacuum.

__________________
Jim