I have never been to a track day, but I can say that I normally replace half worn tires with new before I go to a FJR rally so that I will have fresh rubber and not have to worry about it during the rides. Last year I changed my habit and attended EOM with a worn tire. It had enough rubber to ride on and get me home without wearing through but it was just a little more slippery in the wet sections than a newer tire would be, and it always had me thinking about that back tire. So, in order to get the best experience and most fun out of a track day I would start with fresh tires.