Yes, I would do this.
It would be sad to see Smart go, but we have a team that is filled with young quality guards (Thomas, Bradley, Rozier) and there are three outstanding guard prospects in the draft who all probably have a significantly higher ceiling then Smart (Murray, Hield, Dunn) so it wouldn't be difficult at all to replace what Smart gives us.
Finally, Nerlens Noel may not have a huge amount of offensive potential, but at 21 years of age he is already possibly a top 5 defensive big and is already talented enough on that end of the court to be a defensive anchor for a contender. He has the potential to one day be the best defensive player in the NBA.
On top of that is that he is not only a great defensive player, he's also an incredibly versatile one. He has the height and length (7'0" and 7'5" wingspan) to defend against the biggest of NBA players, and he has the lateral quickness and athleticism to stick with quicker wings on the perimeter. That type of defensive versatility is incredibly valuable in this league.
Add the fact that he can hold his own on the boards combined with the fact that he should improve to some degree offensively (he actually shot similar percentages in the paint and from midrange as Okafor did this year) and there's no way I could say no to that.
I love Smart as a player, but his strengths are his intangibles rather than his actual talents. He's a poor shooter, he's an average passer and ball handler, and while his defence is very good it isn't as 'game changing' as Celtics fans like to indicate. Noel is a significantly greater impact player on the defensive end then Smart is.
Smart has potential to become a good player sure - that's why the Sixers might actually consider the idea. If he didn't have that potential, they would laugh at us.
I'm one of those who strongly believes Noel has more upside then Okafor does in terms of his ability to be an all-round impact guy.