qemu-img convert a overlay qcow2 image into a entire image

I have a base image file "base.qcow2" and a delta qcow2 image file "delta.qcow2" whose backing file is "base.qcow2".

Now I use qemu-img to convert "delta.qcow2" and will get a new image file "new.qcow2" which is complete and equivalent to combination of "base.qcow2" and "delta.qcow2".

In fact, i just want to convert the delta qcow2 image file and get a new delta overlay qcow2 image file. So the "new.qcow2" is not what i want. I have to admit that this is not bug. Could you please take this as a new feature and enable qemu-img to convert images in-place?