Comment 2 for bug 960017

I can confirm that a flow text that was flowed into a shape can't be unflowed back into its original rectangle. To do so, we would need to remember the previous flow shape (we currently just replace it with the new shape).
But since Inkscape's flow texts are not based on a valid SVG element (see https://bugs.launchpad.net/inkscape/+bug/167335, "flowed text has no SVG 1.1-compliant fallback"), it's very probably a "won't fix", at least until we have a new official SVG element dedicated to text areas.