If you do this, you must educate users that IMMEDIATE Internal Transfers are no longer possible (they don't have any initial demand) unless they UNLOCK them after creation to add the initial demand.  

They can still do PLANNED Internal Transfers without any issues.

One method is to create an Automated Action to block over picking:

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Code:

if record.product_uom_qty < record.quantity_done:
  raise Warning("You can't transfer more than the Initial Demand!")

Note that this is a code snippet only, and you may need to modify it to be more robust and to fit any specific edge cases you encounter.

For example, if you use scanning or show detailed operations on the transfer type you may need to create another automated action to handle security on Packing Operations (stock.move.line)