Verified on qemu-kvm-0.12.1.2-2.375.el6.x86_64:

1.
[root@localhost ~]# qemu-img create -f qcow2 base 10G
Formatting 'base', fmt=qcow2 size=10737418240 encryption=off cluster_size=65536 

[root@localhost ~]# qemu-img create -F qcow2 -f qcow2 -b base sn1 10G
Formatting 'sn1', fmt=qcow2 size=10737418240 backing_file='base' backing_fmt='qcow2' encryption=off cluster_size=65536 

[root@localhost ~]# qemu-img create -F qcow2 -f qcow2 -b sn1 sn2 10G
Formatting 'sn2', fmt=qcow2 size=10737418240 backing_file='sn1' backing_fmt='qcow2' encryption=off cluster_size=65536 

2.
[root@localhost ~]# qemu-img rebase -F qcow2 -f qcow2 -b base sn2

[root@localhost ~]# qemu-img info sn2
image: sn2
file format: qcow2
virtual size: 10G (10737418240 bytes)
disk size: 196K
cluster_size: 65536
backing file: base

3.
[root@localhost ~]# qemu-img rebase -F qcow2 -f qcow2 -b "" sn2

[root@localhost ~]# qemu-img info sn2
image: sn2
file format: qcow2
virtual size: 10G (10737418240 bytes)
disk size: 196K
cluster_size: 65536

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

http://rhn.redhat.com/errata/RHSA-2013-1553.html