Redmine

We have groups of employees specialised in different categories of issues while end users submit issues via mail (and are unaware of categories).

We would like our project manager to have the ability to default assign an employee or group automatically when he changes the category. This default assign should occur AT LEAST when there is no one assigned to the issue when category is changed...

This feature is implemented in the "New Issue" workflow but I basically want it for issues that were already submitted but without categories...