If it is your contention that Article 19 gives you carte blanche to demand an upgrade anytime the below occurs then, well, we'll just have to let that stand as is and let folks form their own opinion.
The carrier is liable for damage occasioned by delay in the carriage by air of passengers, baggage or cargo. Nevertheless, the carrier shall not be liable for damage occasioned by delay if it proves that it and its servants and agents took all measures that could reasonably be required to avoid the damage or that it was impossible for it or them to take such measures.
http://www.jus.uio.no/lm/air.carriag...montreal.1999/