If you have the integrated suite (upstream/downstream) the buyersystemid can't be updated. Otherwise you can update via the Supplier Org profile Actions edit org ids or via CSV import
TY Maria again, :-)
I'd like to do it by mass upload, so by CSV import.
do you know which import I have to use ? data import / export, but after which import .?
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