down vote

I recommend before doing any of this you take a complete copy of the disk image as it is, then when it all breaks you can simply copy it back to start over.

There's 3 things you need to do:

1) Make the disk image bigger. In your host:

qemu-img resize foo.qcow2 +32G

Now your guest can see a bigger disk, but still has old partitions and filesystems.