Vendor list and preferred vendor

Hi,

We are using classic scenario and currently, all carts are intercepted by the purchasing department so purchase requisitions are manually converted into POs.

We want to implement vendor lists but SRM assigns any vendor chosen from a vendor list as ‘fixed’, which means it will convert to a PO once the shopping cart is fully approved.

Is there a BADI I can use to change the vendor from 'Fixed' to 'preferred'. Can ‘BBP_DOC_CHANGE_BADI’ be used to do so or does it only works when changing a 'preferred' vendor into a 'fixed' vendor?

Thanks