You could create a new standalone hvm using the template as a source (in
the dialog, click standalone first, then hvm, then select the template).
If you're on btrfs, Qubes will create a reflink from the original disk
images, so it will be instant and take no extra space.
If not on btrfs, then it will duplicate all the data from the template
images. To avoid this, you can create a regular hvm (click hvm in the
dialog and standalone should grey-out) then manually move the root.img
and private.img files from the template to the new hvm.
Chris