This is not an eqipment change.

The have shrunk the business cabin to make Y bigger. They believe they are more likely to sell the additional Y seats than the J seats.

Curtain moves are very common, making ce bigger and smaller

The curtain could move again if they sell more CE seats

Seating within a couple of rows either side of the curtain you are at risk of being moved. Its called curtain roulette

its perhaps a little surprising if gone immediately from 9 rows to 5, but it may have gone from 9 to 8 to 7 after you booked.