In open gl canvas you can simply use "e" to open the track properties. There you can move it to whatever copper layer you want. (It does not create vias though. That needs to be done by the user.)
You can even select multiple track segments by use of the shift key and left mouse button click and use the track properties dialog to change the layer of all of them at once.
You can even use the block select tool for this selection process.
Simply press e again while your tracks are selected to open the track properties dialog.
I tested this in kicad 4.0.6 under fedora.
Switching to open gl canvas is done via the F11 key or in view->switch canvas to open gl
Another option is to use the auto delete track tools of the open gl canvas and just redraw the track as you want.
This button in the left toolbar controls if auto delete is used:
- With the track tool open, left-click on the point where you want to start the move to the other side (This starts a new trace from that point)
- Press v to get a via, place the via where the surroundings permit it,
- Draw the part of the track you want on the other side (You might not always want to have it following the exact path of the old trace)
- Press v again to get back to the previous side. (again place the via where the surroundings permit it.)
- Connect the via back to the original trace. The old trace should be deleted automatically.