What is your motivation? Try to convince yourself first because it is another 4- or 5- year commitment to research in a different field. I dont think you need a master in economics, given your prior background. However, what is your mathematics background? (Such as real analysis with topology space, stochastic analysis, measure theretic probability, optimisation theory, etc.)
To which field of economics? Why do you think you will not change your interest again since you have changed once. (You dont need to answer me the question but make sure that you are well prepared. Your future supervisor may ask you this sort of questions.) I think your math background is OK. In any cases, you will be taught some specific math courses in your first year.I am sorry that I can not offer you information about economics program because I am not economics student. (My field is financial mathematics.)