I wish did fix the issue. Based on my knowledge, it is not possible. You could check managed disk rest APi. Azure does not provide such parameters. So, you could not use Azure Power Shell to get managed disk usage information, you only could get total space.
Azure/PowerShell : How do I query disk state? (Azure Managed Disks)
seems to work fine The short answer is yes, the managed disk would not show the URL that you need to download from Azure through PowerShell, even if it is also just a VHD file. So you should first get the VHD file URL through copying it to a storage account blob. Then you can download it with the ways you want. I suggest the AzCopy that provide in your link. Or you can just generate URL and then download it in the Azure Portal without copy it to a storage account first. Hope this will help.
Conversion time of a regular VHD disk to a managed disk
To fix the issue you can do There aren't any published SLAs for conversion and the time taken will vary depending on VHD size, time of day, region in which you are storing the managed disk etc. As a very rough estimate, a 240GiB VHD stored as a Blob should take in the region of 1 minute. So we're talking in seconds and minutes rather than hours.