[Courtesy: Microsoft Knowledge Base Article]
As mentioned in the subjected above, we have encountered the issue that existing alerts were not firing after the site migration. I digged into the Micrsoft Knowledge Base and I found out the below support article.
There is an awesome tool for SharePoint administrator to update these existing alerts.The new stsadm -o updatealert command was released in the SharePoint Administration Toolkit. For more information and to download the toolkit, visit one of the following Microsoft Web sites, depending on the version of Windows SharePoint Services that you are running.
SharePoint Administration Toolkit x64
SharePoint Administration Toolkit x86
For more information, see the “Microsoft SharePoint Administration Toolkit” white paper. To do this, visit the following Microsoft Web site: