So who reports to you now doing the job you are being asked to do in the future? Depending on the answer then it doesn’t necessarily sound to me like the new person being brought in is being asked to do exactly what you were doing?
Ask to see the respective job/role descriptions, but this will probably not make you too popular. A good chance they do not exist.
Best advice would be, to be professional, ask for clarity on the two respective roles and why the decision has been made. Suck it up, give it some time, see what you think and then try to find a suitable internal move (Big Corp). IME in corporate life there are successful approaches and not too successful approaches to getting the senior management to support a move to develop your career etc