Thanks all. Looks like I will keep the existing arrangements. Regarding the cancellation issues, I wouldn't mind being disrupted and transferred to JAL J (one can dream) but overall it would be an extra hassle and eat into our time in Tokyo.

The only temptation about it all is that I have an upcoming (and past run) of CW trips - typically luck that CW has been rostered on almost all of my flights - and 14 hours in CS was a bit of a draw. But perhaps not at the expense of feeling like a zombie in Tokyo in the early morning. I will recross my fingers for the unicorn that is 2x F reward redemptions.