I don't think the Grant of Rights itself is vulnerable. If Oklahoma left the Big 12, for example, the GOR would still be in place; the networks would still own the rights to Oklahoma's home games; the conference would still be paid for those rights; and, in theory, Oklahoma would still be due monies owed for those home games. Payment has inherently ALWAYS been the 'consideration' in a media deal with a conference and its television partners.
The crux of the issue here isn't Oklahoma breaking the Grant of Rights. Simply leaving isn't breaking anything. The Grant of Rights isn't violated if they leave. Where the problem arises is if the Big 12, arising out of its altered bylaws, attempts to withhold monies that are otherwise owed by the Grant of Rights. Then Oklahoma would be forced to litigate against the Big 12 for violating the Grant of Rights. You'd essentially have two competing authorities (the signed Grant of Rights and the bylaws).
The Grant of Rights itself wouldn't be the obstacle to Oklahoma leaving. The Big 12 knows that, which is why it had to change its bylaws. That's where we go into uncharted territory.