Yes it will come out a little bit. If you swap them straight, I assume you will allow some air into the system. Draco was helpping me by slowly pusshing the brake petal to push as much air out while I am "plugging" in the new switch.
He also helpped me to install the new BLASAS (the new device to protect my new switches. We took some pictures and short movies but I don't have time to load them up yet. The thread below my help you some ideas.
http://forums.pelicanparts.com/showthread.php?t=390422&highlight=brake+light+switch