Given their pricing model and customer base, I don't think their J class command a premium over Y per sq (ft/m) so they are probably indifferent to the mix of J vs Y as long as the whole plane gets filled. Aircraft refreshment will also mean in the interim (could be months or years) there will be planes with different seat configurations, which presents operational challenges if you try to use a plane with low J on a flight sold with high J (need to downgrade J passengers), or use a plane with high J (low Y) on a fully sold low J flight (some passengers will be left behind).