Upgrade Smalling, keep Blind there. Yep, I am serious.
Even more serious though, get a very good CB (where do they grow) and then rotate Blind and Smalling for the other position, and give the spot based on who we play and which one of those two play better.
For what is worth, I think that Blind has had a better season, though Smalling at his best (August-November when Caf proclaimed him as the best CB in the multiverse since at least big bang) is a better defender than Blind at his best. Smalling on the ball ability is criminal for a professional player though, probably the worst I have ever seen.
If we get Mourinho, I think that Smalling will be favored and Blind will be played in other positions. In the ball ability is far less important for him than being a big strong defender.
About the thread, I think that for the majority of time this season, they have been living in completely different frequencies. In some games it looked quite bad, like Blind being in no man's land, or Smalling going for a ball and falling on his arse. Today it was quite better though, and they looked like a real partnership.