On June 30th, 2019, the classic alerts and related APIs for Azure Monitor will be retired.
If you are using this Azure classic capability, you will need to migrate it.
Luckily, a migration tool has been provided by Microsoft to let you do the migration – or if you are confident and don’t want to it by yourself, let Microsoft do the migration job in July (meaning this will occurs after the retirement date and you may have a period without any notifications).
You can read more about the replacement here https://docs.microsoft.com/azure/azure-monitor/platform/monitoring-classic-retirement
To start using the migration tool connect to your Azure portal (https://portal.azure.com) and reach out the Monitor\Alerts blade
Then click on View classic alerts
It will then list all classic alerts you still have in place and will tell you if they are ready for migration
You will then be able to preview the migration and validate the results before executing the migration for say.
During this process no modification can be done on these alerts but they will continue to be fired based on their conditions.