To answer your question, it depends on the bag. Some are secured with straps and clips, and you have to unclip a couple straps and move the bag to the side to access the tank.
Others are magnetic, and you have to simply lift the bag off the tank and move it to the side.
My tank bag (Chase Harper) is both magnetic and has one forward strap (I paid about half that 2-3 years ago for a new one). My FZ6R is the first bike I've owned that I don't like riding with a tank-bag. I'm not really sure why, but it bugs the crap out of me; I felt like I had reduced my manueverability...hard to explain. Anyway, the few times I rode with it on the front, I simply had to lift the magnets away from the tank and move the bag to the side.
It secures nicely on the back, although it is a little tougher to remove. Fortunately I really don't have to remove it for anything, and once I strapped it on back there, I never took it off.
Pics to follow.