On ND's athletic site, Elston is no longer listed as assoc. hc. So, yes, like I suspected, he and Polian apparently, just switched roles. So, I guess, being named running game def coordinator is considered a promotion over associate head coach.
The bottom-line is he is likely getting a good bump in pay, and the extra responsibilities of what is essentially a co-defensive coordinator title, will make him more attractive as a head coach candidate. That, I am assuming, is what would be the most important aspect of the move.