Mountainguy,"good on you" for trying to handle this in a politically sensitive way. I was not as savvy about such things when I was in grad school. This bodes well for your future in dealing with the petty politics of academic departments!
I suggest that you approach your current advisor for advice. Even though you have made up your mind, you can frame the questions as if you haven't. "Prof Prickly, I'm at a crossroads. I've always had a strong interest in areas B and C. What if I..? Here's why I think it would be a good career move.. what do you think? Would it be better if I moved to a different institution? If I stayed here, do you think Prof Good or Prof Nice would be a good advisor?"
Your prof will be flattered, rather than blindsided (as s/he might be if hearing about your decision after the fact). Remember, even if you don't want to take the proffered "advice", you can nod sagely and thank him/her profusely, and promise to consider it.
But there will be people in my department who will question my motives, since my old advisor has a reputation for being somewhat obnoxious.
As tempting as it might be, please try not to discuss any issues you had with Prof Prickly. It's never too early to start racking up the collegiality points!