I don't find seat changes on BA happen all that often. I can probably count on one hand how many times it is happened.
Only couple of times I can remember is when the curtain moves but this does not typically affect the emergency exit rows.
I have never been shifted on a longhaul, except for Op-Ups, but that is not surprising since I only use BA for JFK/PHL-LHR on any sort of frequency.
If you are precious about seating, I urge you to avoid LX/LH where you will find a very succinct piece of paper spluttered out at you from the auto-boarding gates with your new seat assignment. This happens frequently on their "shuttle" routes (e.g. FRA/MUC-ZRH).