@HBK The disk type conversion is instantaneous. You can start your VM after the conversion.
when you are resizing the disk, 2 parts are there now:
1 Size change in terms of actual space like 256 to 512
2 You will keep the 256GB disk size the same, but you can increase its performance to the same level of p20 (512) disk.
So I believe you are referring to the actual disk size change. In that case, there should not be much issue unless he encounters any allocation failure while starting or deallocating the VM and post resizing the disk from the portal, then you need to expand the disk from "Disk Management" in the VM.
Additional information: In most cases, the desired new VM size is supported on the current hardware cluster hosting the VM. In this case, resizing a VM should not take more than a few minutes, since Azure only needs to change the metadata of the VM and allocate the new size. These are only administrative host actions. In these cases, the Azure only need to restart the VM if it was running.
Hope this helps!
Kindly let us know if the above helps or you need further assistance on this issue.
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