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.
- Go to Item Prices sub-tab.
- Set it into Edit mode and delete whatever price you have entered.
- Save the prices.
- Click to say Custom Fields.
- 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.