Is there a way to enable users to change unit of measure on a material when doing GR, GI and placing an order?
Scenario: Buying goods in a box, and issuing in packets or each
Where can I make these changes?

The only change that will affect your UOM is when you place an order.(GR/GI will consider what has been captured when an order is placed). Bear in mind that the UOM is dependent on the type of Order that you are placing.

Internal Order will always check for Base Unit of Measure, whereas, External Order will do the conversion if you use Preq.

If you place External Order directly on ME21N, the system will automatically pull through your Order Unit.

Hope this will make sense. ( All this changes can be done on MM01 / MM02 )

Purchasing View.

Plant Data View.