Requires a Business rule to Clear the "Assigned to" field when the Assignment Group changes ?

[sriharshabalsu]

In an incident form whenever the "Assignment group" changes the "Assign to" field should be cleared, if that assignee doesn't belongs to that group. I required a business rule for aabove requirement.

[morrix7]

What would be the best way to make this happen but onChange of the assignment group value and not when the record is updated with the new group?