I think the point of invoking Rule 240 is that airline agents will often try to deflect passengers' reasonable requests for re-accomodation by claiming that they "can't do" something-or-other. When Rule 240 makes it clear that they certainly can, the issue becomes whether they want to bother to do it or not. Since you've now proven that they've either just lied to you, or, at best, been mistaken, what was impossible a moment ago may have just become possible.