Create a Server Action like this:

for record in records:
  product_template = env['product.template'].search([('id','=',record.res_id)])
  for variant in product_template.product_variant_ids:
    new_attachment = record.copy()
    new_attachment['res_id'] = variant.id
    new_attachment['res_model'] = 'product.product'

And make sure to click CREATE CONTEXTUAL ACTION so you can select the relevant attachments and have them copied from the product.template to the product.variant:

Then you can re-select all the attachments and DELETE them (if applicable).