The following is the description of the WT_PREPARE_CONFLICT.
Conflict with a prepared update.
This error is generated when the application attempts to update an already updated record which is in prepared state. An updated record will be in prepared state, when the transaction that performed the update is in prepared state.
**This error never occurred in my test with update of a record that is already updated by a transaction which is in prepared state. It always produces the error of "conflict between concurrent operations". But the WT_PREPARE_CONFLICT error is raised when the record is read and it is true also as per the description in the following link.
The documentation of the error needs an update from update to read in the following sections