Then perhaps your best option for now, until all the aircraft are fitted is to select the exit row by preference and avoid this whole situation. Once the entire fleet are fitted then you won't have this problem.
Yep - I know that now!
By the way, I have since successfully experienced YX. This time using Fly Ahead - I got 5A. The lounge agent made a point of saying it's an extra legroom seat, to which I replied "are you sure?" and told her about my previous experience. She said something along the lines of "oh, that's a shame, but this plane definitely has Economy X'. So either this info was available to her, or she was just bluffing! Anyway, the legroom was good - definitely noticeably better than the old row 5. Rows 3-5 were completely full, so no shadows. Still though, Fly Ahead on a busy flight often used to mean getting seated somewhere down the back, so this was a pretty good outcome.
Going back to the previous discussion about whether Economy X might persuade me to fly more often with VA again - I remain undecided TBH, as a result of my mixed experience this week.