Yes, sadly. Its only in the days close to departure that you really know how many you have in the J or F cabins as more in these cabins book within the last 14 or even 7 days. I have had a LAX rotation cancelled on the day because of it, we were readdomodated 2 hours later so no great shakes. (Our aircraft actually did depart for LAX but with no passengers so probably had a rich cargo belly or was needed to operate the return, and it was just better to run for BA to run the outward with no passengers).

As you rightly say though Simon, they tend to do more of these cancellations a few months out these days, to minimise inconvenience, but they do occasionally happen.