Find the right accounts and cases
Report type = Account with Cases
You need the Account IDs and the Case IDs to do the reallocation, and the upserts will be separate jobs.
You also need the user IDs (Salesforce IDs) of the users you will be re-allocating to. You get them from the user record in Salesforce (from the URL, see the highlighted section below, if you double-click on it, then it will be selected for you).
Prepare the upload file
You should have two columns, as below...
...now replace the Account owner values with the IDs of the users you want to re-allocate to, like this...
Make sure your files have no extra rows or columns. Then save the file as a .csv.
Now repeat the above with Case IDs and Case Owners.
Open the workbench
If you are logged into Salesforce, it will allow you straight in.
ai) Set the operation and the object. Upsert is the correct action (a mix of "update" and "insert") as we are making updates, not creating new records.
aii) Select the file in the usual way, then click "Next".
aiii) Map the fields
Id --> Account ID
Owner --> OwnerID
...scroll down to the bottom of the page, and click "Map Fields"
aiiii) Final checks then run the job ("Confirm upsert")
N.B. it is best practice to always select the asynchronous option.
Exactly the same as the above, but select the Case object when setting operation and object.