Hi,
FERT material type of material and your expected material type, both have different views where as for your expected material do not need accounting views.
Yes, You can change material type of material in t.code MMAM but both material types characteristics and nature are different - You need consider this as mini project based on quantum of materials available and material type to be changed. You can go for new movement type creation and follow REVERSAL of stock integration/ reintegration process
Regards,
Biju K