To transpose just a section in Cakewalk, I tend to split the track(s) into clips at the points at which the transpose is to start / end. This is more precise than just selecting all the tracks and then subselecting a timezone, because sometimes you want to transpose a note that starts before the transpose of other tracks.

Having split the tracks, select the ones you want to transpose (ctrl-click) and use the Edit-Transpose facility.

And rememeber not to transpose drum / percussion tracks!