Hi,
To work out the VMs that need migrating you should be able to look in Azure Advisor and I would expect that the VMs will be listed. You can also go to the Virtual Machines page and filter on Size.
As for the retirement MS are upgrading the underlying infrastructure and therefore getting rid of the older boxes. So the need for the upgrade is to move off the Av1 series boxes onto Av2. More info can be found here: https://learn.microsoft.com/en-gb/azure/virtual-machines/av1-series-retirement
Hope this helps.