Indeed it would be easier to deal with this type of geometry using [Bezier Curves instead][1].

Otherwise just use the *Bridge Edge Loops* operator.

Erase the straight corner faces, select both end edge loops, call the *Bridge Edge Loops* from the Space bar Menu, then adjust the number of cuts and smoothness as desired.

[![Bridge][2]][2]


  [1]: https://blender.stackexchange.com/questions/66353/how-to-make-beveled-90-degree-corners-on-a-curve-better/66367#66367
  [2]: https://i.sstatic.net/k6QPh.gif