How do place cells switch their response patterns when the subject moves to a different environment?
Imagine a 2x2 grid. Starting on the bottom left, how many different ways can you move to each box of the grid? 

Two times: 1) up, right, down; 2) right, up, left

Now, say each of those combinations are used to store the memory of a different location. In order to activate either sequence, a different input signal would be required to tell the axon to send it's initial signal right, or up.

Your eyes will receive a signal in the form up light, and a specific electrical impulse will be sent down the optic nerve that allows your brain to perceive an image. 

There are millions of nerves in your brain, and each combination of impulses serves to store a different memory. 

Now, imagine a grid that has a million by a million boxes. How many combinations can you use to fill each box?

And, actually if I recall correctly, the hippocampus is one of the few areas in the brain that is consistently undergoes neurogenesis.