Call your local building department. It has sole authority over whether the conversion can be done. If you do not live in a municipality, call the county or parish department of building and zoning.
Schedule a pre-inspection if your building department allows it. In some cases, whether it is allowed depends on where your meter base is. If it cannot be adequately secured from children, the inspector may deny the request even though it is not completely banned.
Follow the instructions the building inspector gives you. In most cases, for existing structures, the building department will require a certified electrical contractor to perform the conversion.
Check with your local electric utility company. In some cases, it prefers to do the conversion itself. Before agreeing to that, check to see what fee it will charge and compare it to the price you would pay for an electrical contractor.
Schedule the conversion with the contractor or utility company. When the conversion is complete, let the building inspector know so he can do a final inspection.