Other than on the day of travel, it is the ticketing carrier's responsibility to handle a reroute. On the day of travel, that shifts to the operating carrier. Thus, the "whom do I contact" question depends on when the flight is delayed or cancelled.

For this particular route, it really will not matter. DL handles customer service for both carriers in North America and AF does the same in Europe. The route is covered by a JV. Simply a matter of which toll-free number you dial. You wind up with what amounts to the same agent.

As always, if this occur, you should have handy the reroute you prefer so that you proactively seek what is best for you.