According to the BA website, you are unable to change the dates of reward bookings online from the 21st April onwards (
https://www.britishairways.com/en-gb...b/club-changes).
I had to make a return leg change from NRT last week and the 3 available reward seats went within a couple of minutes of waiting online. Luckily, the Exec Club operator managed to get me a couple of returns from HND even though they weren't showing online. Switching reward flight dates are going to be more of a hassle now if the policy doesn't change back but it's still better value to try for that in most cases. Just be prepared for early morning phone calls (may get lucky with the US number?).