You cannot change the exam centre,after the submission of the exam form in IGNOU.As in the form it is mentioned in the last once you submit your form you cannot change your centre.

But you can try by going into your regional centre provided by IGNOU. Demand them to change your exam centre.They will give you one form,you have to fill all information in that form like your enrollment number, present centre,desired centre and reason to change,Your IGNOU ID card.And after that if the authorisation give permission, they are bound to change your centre.