It's always a crap shoot unless you have some flexibility. I booked on AC07 for next Friday when it was absolutely wide open in J. Now AC07 and 15 are down to J3/J2, which is out of my comfort zone for Aerolotto. But the Thursday flights had plenty of R space. If you have the time and you're willing to pay the (only$50) change fee, you can improve your chances significantly.