I agree with Mike, but it's also perfectly OK to say "What was it before?" This is probably what I would say, personally, but it's slightly more formal than "What did it used to be?" "What was it formerly?" is too formal. It's grammatically correct, but it sounds stiff.

'What did it used to be' is the best of the options.. - I changed my name 2 years ago. - Really, what did it used to be?