There is indeed an equipment swap today for the AF010/011 rotation from a B77W configured 4P - 38J - 28W - 252Y to a "Quick change" config B772, i.e. 16J - 24W - 276Y. So obviously a lot of downgraded people tonight from P to J, from J to W and from W to Y (your case).
You are certainly entitled to change your flight, to stay in W, but keep in mind that the flights are very full now and so it might be difficult to find 3 seats today in W.