Hi Sana,
Yes you are right. It is possible to change the material group of a material. But from a business perspective it is not reccomended. Material Groups categorize a material. Instead of changing the material group, it is beneficial to create a new material with the revised material group and setting a deletion flag for the old material.
Again, by setting deletion flag for a particular view is not suggested as inconsistencies will creep in. Before setting the deletion flag against a particular view, we will need to check whether there is any stock available for the material, any open orders against it. Otherwise it will be very difficult to clear such entries at later stage.
To avoid these implications only, i suggested not to change the material group or delete a maintenance view.