What AA told you is correct. They sometimes have access to partner seat maps but that's not always the case. Your best bet is to get the JAL an CX confirmation numbers and select seats on their websites.
If you'd like to change the dates, you can certainly do that but if you don't hold status with AA, there will be a change fee involved and since we're talking about 4 seats, you'd have to pay 4 change fees. So, keep that in mind. Good Luck!