One thing I can recommend for these situations is a subscription to Expertflyer. For every flight that I book I set an alert to notify me the instant that:

- There is an AC change
- Schedule change (or cancellation)
- Upgrade space becomes available
- Better routing (if any) becomes available at a similarly priced fare

Especially now with the flexible (re) booking options this has proven quite valueable. For instance I generally prefer morning flights out of AMS, but had to book an afternoon flight recently because the morning flight only had full-fare available. Set the alert and a few weeks later the notification came. Changed the flight for free. Most likely one person cancelled their flight and as I was notified immediately I was able to grab that one single seat.

Expertflyer does have it limitations though, of course, for instance the attitude of their sales/support team is not quite what you'd want. For instance they allow setting alerts when specific seats open up on your flight. But not when a specific seat is being taken (e.g. the middle seat in a row of 3). They are unwilling to look at anything that may improve the service, and as a result their website (and features) have remained the same for many many years. Another thign that is deerly missing is tax information (Especially the bogus airline-added fuel taxes and such).