Hi Hugo,
I will not suggest to change the buyersystem ids in mass through CSV as there are transactions in the system which are related to these vendors. If the buyersystem IDs are changed these transactions will be impacted.
Also, you need to check that there are 3 level of supplier hierarchy which need to be uploaded in correct order. Check for SupplierIDs.csv and SupplierOrganizations.csv
Regards,
Ravi Pachauri