You can animate the zoom level of the camera if that's what you want to change.
I was playing with the zoom levels of different camera presets but the levels applies to the entire composition only.
What I want is to have the 50 mm preset until a certain time frame and then change it to wide angle 15 mm once the
cube middle point is reached.....
Select your camera layer and press the A key twice. Now click the stopwatch on any of the properties you see including zoom level. A standard 50mm lens has a zoom value of 2666.6667 pixels. Once you have enabled animation of the zoom value you can either drag the value around in the Timeline or you can double click on the camera and change the lens settings there. Your project should look like this:
By the way, changing the zoom value or more correctly, focal length of a lens does not change the perspective. Perspective is controlled by camera position. If you want to change perspective, change camera position, if you want to change the framing then change the focal length. AE's camera is 100% optically perfect so you won't get barrel distortion with a wide angle lens like you would with a real camera so if you want to simulate or match the perspective of something like a GoPro with it's nearly fisheye lens you'll have to add Effects>Distort>Optics Compensation and set a value for field of view.
Here's a CS6 project for you to look at. Note that the perspective does not change until I move the camera. When you download the file your Browser may add a .txt extension after the .aep extension. Simply delete the .txt and AE will open the file.
Thanks Szalam, Rick, you were both correct while Rick explained more like a newbie would understand. I was thinking too simple- like real life, take the 24mm film/camera lens for the entire pyramids shots, or the 400mm for wild life shooting or the 1200mm for the moon shot.... happy holidays to you.
I'm glad we got it figured out! Seasons greetings to you, as well.