Also bear in mind that point of departure and point of sale make a difference. Thus, ex-UK I fares are non-changeable whereas ex-EU can be changed.

I am a bit surprised that an I fare gave an upgrade to A one-way. Normally BA restricts these promotions to higher class base fares.

Finally, for ex-EU feeder flights, the SH will normally book into J. This does not give fully-flexible J rules - the fare rules of the overall ticket apply. Instead BA use the J bucket to maximise availability of connecting flights as they want to sell the LH ticket. The fare bucket alone is pretty meaningless for changes - it's the fare rules that matter.