Hmm I don't change the day before but I do change as soon as I get on the plane. Sometimes it results in an extra meal and sometimes it results in one less meal but for me personally, eating to the destination time zone goes a long way towards preventing jetlag, especially on TCONs (which I'm on fairly often). It's important on TATL too but less so on TPAC where the change in time zone is more dramatic and shifting eating times alone isn't enough to quickly adjust.