Given it's an NTFS file system, you are essentially moving a subfolders/files to a parent folder in the same volume so a copy operation is not required. The most efficent method in this scenario would be to simply move the data. You could do that in windows explorer (assuming you create a CIFS share to the root of the volume) traverse into the qtree (CTRL-A, CTRL-X, browse back to the root of the volume and CTRL-V) then delete the qtree but you might also want to consider robocopy.exe with the /move parameter as a more robust solution. I would advise deleting the CIFS share at the qtree level to ensure clients can't write to it whilst you are attempting to move the data and you might also consider temporarily disabling snapshots on the volume.
Also you could easily test this scenario by taking a flexclone of the volume first then moving the data as mentioned above. This will enable you to non-disruptively test your change without consuming any additional storage capacity.
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