There have also occasionally been issues with Virgin Atlantic award seats disappearing from inventory for a little while because they were "viewed" through the online engine and then seem to get held by that engine and aren't available for someone else to book. Not sure if this has been happening with the new booking engine, but it was a frequent problem with the old engine.

I would suggest waiting until the next calendar day and calling (without doing any searches online first, so as to avoid putting any seats on hold inadvertently) to request the change, just in case.