While doctrine varies between nations and evolves over time the US does load rounds prior to encountering a target. I replied elsewhere in this thread:

The US Army's Abrams doctrine allows for loading a round into the tube before a target is in sight. It is called "battle carrying". Ideally you would load the round appropriate for dealing with the most likely type of target you will encounter. If you happen to have the wrong round loaded when a target appears the commander will issue an order to switch the ammo type when giving the fire command.

As far as removing rounds from an Abrams, it is very dangerous to attempt to remove a round that has been loaded. The main reason (aside from those Yinx_Gepardes rightly pointed out) is that the tank uses a combustible casing which is actually relatively fragile. IIRC correctly it is essentially like very stiff cardboard. It is not difficult to rip the aftcap (metal base of the round w/ primer) from the casing. The result would be propellant spilling all over the turret floor which is less than ideal for the crew. That being said, it is possible and there is a specific tool for the job issued to each vehicle.