While all NFL defenses play around with formations depending on the situation, every team has a base defense that functions as their identity and the starting point from which they can build out. The base defense is key in determining what types of defenders you bring in, as well. For instance, you might not want to bring in a standup outside linebacker if you run a base 4-3, in which he’ll be required to put his hand in the dirt, which could affect his pass rush arsenal. 
Well, according to Jeff Duncan of The Times-Picayune, the Saints are officially switching to a base 3-4 defense under new defensive coordinator Brandon Staley. Staley learned the scheme from Vic Fangio during their time together in Chicago and Denver, while Fangio was a student of it all the way back during his first NFL gig as the Saints linebackers coach in the 1980s, when the imposing “Dome Patrol” quartet gained notoriety in New Orleans. But the 3-4 went away from the Superdome when then-head coach Jim Mora resigned in the late 1990s, and the defense has been mostly run out of a base 4-3 since then.
For the most part, the responsibilities of the secondary will remain unchanged in the new formation. The base defense will still include two safeties and two cornerbacks with extra defensive backs coming in for special formations. The responsibilities of the middle linebackers will also remain mostly the same, barring the fact that their new base defense allows for two strictly off-ball linebackers.
The differences come with the outside linebackers and the defensive line. When running a base 4-3, defensive ends are the main pass rushers. Outside linebackers can blitz on occasion, but for the most part, they act more as off-ball linebackers than edge rushers. The defensive tackles, in 4-3, are stout and strong, but they also usually to have a bit of a pass rush arsenal. In base 3-4, the outside linebackers become the main pass rushers, though they should also have the versatility to function off ball. The defensive ends now are more akin to defensive tackles in size and pass rush ability. The nose tackles become more important in the 3-4 as an over-the-center, usually heavier, run stuffer.
In New Orleans, determinations are being made as to which defenders will have what roles. Formerly defensive ends, Carl Granderson, Chase Young, Cameron Jordan, and Isaiah Foskey are now outside linebackers. They’ll have to adjust to working out of standup position, as opposed to the three-point stance they have been used to. In order to adjust to this new starting position, Young and Granderson have reportedly shed some weight.
The new defensive ends group is composed of Bryan Bresee, Jonah Williams, and Vernon Broughton, while the nose tackles will be former defensive tackles Davon Godchaux, Nathan Shepherd, and Khalen Saunders. Their responsibilities won’t really change too much, aside from their new three-man lineup being surrounded by pass rushers from the linebacking corps.
Obviously, Staley and company will constantly be changing up looks and rotations throughout the course of games and the season based on different situations and different offenses they will face. But this change in the team’s base defense alters the role of several key, veteran players and will likely affect the prospective players New Orleans looks for in the future.