I have some F-Stop bags and they are really well made and comfortable. The Mountain Series http://shop.fstopgear.com/us/products/mountain.html
have their access from the back-pad and may allow you to flip it around on your hips and access the gear.
There are a few other options that may satisfy your need to change lenses and not put the bag down. One idea is to use something like a lens change bag that you attach to your hip belt, shoulder strap or gear attach point on the pack.
The side access bags are another idea, but mostly sling type and may be a little small for your needs.
I recently saw a system, the name escapes me, that had a modular system with a main bag in the back and multiple bags that could be added to the front to counter the weight in the back.
Then back to the F-Stop bags, this woman claims she can flip that Loka in the video around on her hips and change out lenses. I don't know why she did not demo that, as that would have been way more entertaining than watching her unpack her bag: http://youtu.be/d-AX1HtbmZk