If your ticket is already issued in 'U' class (award J), I believe that you can be booked in Y class, go to the airport and ask to be put on the waitlist for business. Since your ticket already shows 'U', CX will offer you a business seat if available.

This is based on my past experience on a F class award ticket and I believe will be the same for a J class award ticket.

However, if it was me, I won't risk taking economy over business for that route. business over first maybe, but not economy over business.