This is a condition of low-priority VMs, although they are now known as spot VMs. Microsoft have the option to deallocate these where capacity is needed so they are really only useful for interruptible workloads.
There is no upgrade method for these so you would need to create a new VM as a 'Standard' virtual machine. You can snapshot your existing disks and create a new VM from these.
https://learn.microsoft.com/en-us/azure/virtual-machines/snapshot-copy-managed-disk?tabs=portal
https://learn.microsoft.com/en-us/azure/virtual-machines/windows/create-vm-specialized-portal