how do I mass update account owners and contact owners?
Take note of the information on the Mass Transfer page. When you transfer ownership of an account, the new owner will also gain ownership of the following records related to the transferred account:
- Any notes that belong to the existing owner.
- All contacts that belong to the existing owner.
- All opportunities (including closed opportunities if you select the Transfer closed opportunities checkbox below) that belong to the existing owner.
- All open activities assigned to the existing owner. Note that completed activities will not be transferred.
- The new owner might need to edit sharing.
To have more explicit control over the transfer, you can do this update via the API using a tool like the Data Loader. Your .csv file will need the User IDs of the respective Account and Contact owners, as well as the record IDs. You will also need to separate into an Account update and a Contact update as you won't be able to do both at the same time (at least not with the Data Loader).