Actually, you can remove the bolt assembly without degassing the gun. Normally the valve is closed meaning there is no air flowing up into the breech or flowing out towards the striker to push it back. Although it is probably not a safe thing to do, you can remove the internals (except the valve) without releasing any air.
Also, you probably won't cause that much damage leaving the air connected to the gun. As long as you uncock the gun (hold the bolt back as you pull the trigger and let the bolt slide forward slowly to rest, do not push the bolt forward), the only parts that will be under pressure will be the valve and possibly a regulator if you have one. The spring in the valve is designed to be compressed for long periods of time. Technically, the spring is always compressed even when the gun is not aired up.
Also, the cup seal is probably fine under the increased pressure of CO2. Although I would for safety reasons not leave the gun aired up and degass the marker as soon as you can.
You can lube your gun using oil. I recommend gold cup lube or even air tool lube (meant for pneumatic tools). You can also use tri-flow oil
(get it at lowes/home depot). You will want to put oil on the striker oring. The striker is the heavy cylinder in the bottom tube of your gun. It will have a single oring on it near the front of it. Put a drop or two of oil on and make sure it is evenly coated. A little bit goes a long way. You can also put a few drops of oil into the ASA next time you fire the gun, this will push that oil up into the valve area, making sure that it creates a nice seal.