Symptoms
How to view previous values of a workflow data field.
Diagnoses
I don't believe this is possible after the workflow has overwritten the data field's data.
Resolution
If you're looking to build out logic for future troubleshooting, you could add an event that sets an additional data field to the current value of the data field that you want to troubleshoot to view the data at that point in time, or add a User Task to "pause" the workflow at that point.
The user opted to concatenate a Data Field's data in another temporary Data Field in order to track changes. The data limit of a Data Field is nvarchar(max) in the database, so there shouldn't be issues concatenating small bits of data over a workflow for testing purposes.
