Actually, that sounded correct as written (unless I have a wrong idea about your geometry), because the gravitational energy is negative. If you double the gravitational energy, it's correct to say you are decreasing it.
The part that didn't sound right to me is where you said something about the field decreasing inside the shell.
Ooops! I was figuring the shell is pulling up on some guy standing on the surface of B. But I know now what you're going to say. The net addition is zero inside the shell by symmetry, so the guy on the surface of B weighs the same.
At least, I think that's what you're going to say. (It's not hard to prove I'm not a physicist.)