My HVAC person is telling me that they have to cut the 2 2X4 plates that run along the center wall of my house such that they can use the space as a cold air return. The said that they would only cut between the floor joists and not cut th joist, but from a structural perspective this does not sound right.

Is this the only way to get the cold air return from the bastment to the second floor?

The supply run is going to go from the basement and shoot up to the attic on a back wall and then down from the ceiling.