You can update an existing Approval Single Property by specifying the expression to populate the user picker custom field.
- Click

- Specify the expression to populate the custom field
- Save the Expression
- Save the Approval configuration
You can expect that this updated setting is active on any issue that have not already moved in the status requiring this approval.
Case of issue that are already waiting this approval, the updated setting becomes active on loading the issue or running an Approval Re-indexing.