Originally Posted by
dabac
Consider the circumference of the BB shell. Think about how little of that you might lose if you are somewhat skilled when you cut the ring out.
In short, don't worry about it.
I've done this many times with steel and cast iron plumbing pipes of various sizes. I used a Sawzall - definitely not a fine surgical instrument! Even when I grazed the internal threads, it didn't present any problem at all. Your Dremel wheel will offer much greater control.
It's best to make two cuts with a small space between them. Then, knock out the little piece, gently tap the ends of the big piece inward, and unscrew it.