It is possible!

First you need to select your Camera, then go to the Properties panel, and under the Camera tab, Lens subsection, you can choose your lens type;

However, a perhaps little know feature, is that you can right click it (RMB ) and select Insert Keyframe. Make sure the current frame is one before when you want your transition to happen and do this for the Perspective lens.

Then just move to the next frame and repeat the process for the Orthographic lens.

It is possible!

First you need to select your Camera, then go to the Properties panel, and under the Camera tab, Lens subsection, you can choose your lens type;

However, a perhaps little know feature, is you can animate almost everything, so just press I or right click it (RMB ) and select Insert Keyframe. Make sure the lens type you want to transit from is selected, that the current frame is one before when you want your transition to happen.

Then just select the other one move to the next frame and repeat the process for the Orthographic lens.