You can change the supplier using bulk editing in the Variants module.
Open the product you want to update.
Click the "Variants" module tab.
Select the checkbox next to the variant, or variants, you want to update.
Click the "Edit" button.
In the "Setting to update" dropdown, select "Supplier".
Click the "Supplier" field and choose the new vendor from the list.
Click "Save".
The selected supplier now applies only to the chosen variants.