If it appears that you can not select a different Supplier for your Item it is because you have set prices for this Item.

If you need to change the Item Supplier - you will have to remove the prices first.

  1. Go to Item Prices sub-tab.
  2. Set it into Edit mode and delete whatever price you have entered.
  3. Save the prices.
  4. Click to say Custom Fields.
  5. Now edit your Item header again.

Now you can select a new Item Supplier.

Once you have done that - you can re-enter the correct prices.