Assigned to email alert issue

Add-PSSnapin Microsoft.SharePoint.Powershell
$web = Get-SPWeb "http://myServer/myWeb"
$list = $web.Lists.TryGetList("Tasks")
$list.EnableAssignToEmail = $true
$list.Update()

But it will works only we create the new task, not send the email when we modified the task.

How to solve this issue