If you want to change the several materials old material group to new material group then follow the following steps :- 

Step 1. Find all the materials with old material group either from MM60 or SE16(MARA table)

Step 2. Open Transaction MM17- Select Table as MARA and Fields as MATKL(material Group)

Step 3. Press Execute

Step 4. In tab "Data Records to be changed", copy all the materials you have selected in step 1. Press execute.

Step 5. In NEW VALUES field enter NEW_material_group. Press 'Carry out a mass change' button.

Step 6. Press Save. 

Step 7. Check in MARA table for materials with new material group.

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