I currently use a Track to Constraint to make the camera automatically point to an empty.
In an animation the camera is moved along a bezier curve (follow path).
How could I change the curve and track to constraint at a certain frame to focus on another aspects of the scene?
You can animate the 'influence' value on each of the constraints you want to turn off. You can either animate the values off over a series of frames to fade the constraint's effects gradually or immediately.
The one thing to note is that when the object is no longer under the influence of the constraints it will return to the position it was at before it was constrained and will not stay at it's current position.
To counter this you can either reposition the object after the constraints are off or animate the 'influence' of some new constraints on at the same time as the old constraints are turning off.
Sometimes it is a good idea to put empties inbetween your object's relations. In this case the empty follows the path ('Path' constraint) and the camera is connected to the empty with a 'child Of' constraint. You do that with every path you want to use. The advantage is that you can see all path animations simultaneously without the need for the camera to switch.
Second advantage is that the empties can keep their own 'Track To' constraints, so you don't have to switch several sets of constraints on and off simultaneously!
It might be more convenient to switch the whole 'Child Of' constraint by animating the eye instead of the influence. Just hover over the eye after you switch it on or off and hit I.
