There necessarily will be a trick to this.
If there are two flights 6 weeks out and checked bag fares are: 0800 at £200 and 1400 at £50.
Why won't I book the late plane and ring up at 6am to try for the expensive 0800 flight? If I fail, I catch the plane I booked. Otherwise I get on an expensive service for a dirt cheap fare merely because there was a seat in the same cabin available. On a route with lots of flights a day, booking the last/cheapest flight of the day could work very well.
There may be a rule that says the same or cheaper fare bucket must be available for a switch without a fare difference being charged. It doesn't say that in the PDF though.
Or the semi-flex fares will be set with a baseline fare that means these kinds of games don't make sense.