Dont know about Truex & his sponsorship deal, but my thoughts on it is. Truex is still new to driving the NW series & Maybe they werent willing to put him in a plate track yet. JGR has a commitment to their sponsor of the 18 & 20, so either they were looking for the best guy they could to fill in for Denny, or maybe the sponsor didnt want the inexperienced Truex in the seat. Ulitmately for whatever reason it was, I believe it has something to do with his inexp. on these types of tracks.
Why they swapped rides puzzled me, but what SS14 said makes sense to me.