According to Maor Lipchuk, and based on https://bugzilla.redhat.com/show_bug.cgi?id=827529

There is a need to updated the documentation as follows:

It should be stated that 4.1 Data Centers will create QCOW volumes with 1.1 compatibility
> level instead of 0.10 which was until now.
We should stress that this means that once you upgrade a DC to 4.1 (or above,
in newer versions) you cannot downgrade it again to 4.0 or below.
This also means you cannot detach a storage domain from a 4.1 DC and attach it
to an older DC.

You COULD, however, export a VM/template from a newer DC to an EXPORT DOMAIN,
and then import it to an older setup.


This change is based on the new QCOW2 version 3 image format:
QCOW3 includes a version number increase in order to introduce some incompatible features, however it's strictly an extension of QCOW2 and keeps the fundamental structure unchanged, so that a single codebase will be enough for working with both QCOW2 and QCOW3 images. Newer QEMU versions (1.7 and above) support a new format of QCOW2 version 3, which is not backwards compatible and introduce many improvements like zero clusters, and performance improvement.