Are they going to lay it down right over the field turf? Is that going to affect the field once they remove the grass at all.
more or less, yes. (with a protective sheet under the turf) It shouldn't. George Toma, the legendary groundskeeper, pioneered the art of growing grass on turf fields in the late 80s. In those days, Arrowhead was turf, and Toma would experiment with plots of sod above the plastic stuff. His experiments helped the US win the World Cup '94 hosting battle. His work paved the way for putting temporary grass in, and keeping it alive. I would imagine the tons, and tons of dirt needed for a monster truck or motocross even would be more potentially damaging to the artificial turf underneath than a simple grass field. Here's a shot I took in Seattle of grass on turf: