There are two concurrent changes to the starting line up that occurred at the same time. One is getting Smart in and the other is Morris. I am all for (and have been since summer) starting another big and sitting one of the wings. That big is currently Morris and the sitting wing is currently Hayward but it could be Brown or Tatum as well. I think this is the right way to go in terms of getting another big on the court with the starting line up.
In terms of Smart in for Brown, that seems to be working but Smart is also a very reliable weapon off the bench. I don't think there is necessarily a right or wrong answer. Smart is going to get his minutes when it matters either way. For example, maybe the ideal starting line up is Irving, HAYWARD, Tatum, Morris, Horford. Hayward can take on more ball handling than Brown I think. I guess I just like Smart off the bench and he seems to like that too.