the idea of these white costumes is that they are more than just costume upgrades and are actually some kind of space suits which will allow for the Avengers to meet Thanos face to face. The mad Titan was last seen on some alien world smiling over his victory, so the Avengers will probably need to go to him to deal with the situation.
Or perhaps it's more likely that these are time-travel suits, as this trailer does feature a bunch of flashback footage and there have been plenty of rumors and unofficial photos from the set which posit a time-travel solution to defeating Thanos. It's also possible that the suits are somehow designed to allow for the team to travel into the Quantum Realm... Don't forget that Ant-Man's powers are based around wearing his suit, and travel to the Quantum Realm also requires the traveler to be protected by an external shell of some kind. There's even a line in Ant-Man and the Wasp about a time vortex in the Quantum Realm, so it's entirely possible that the Avengers are travelling through time and space via the Quantum Realm.