So I understand that place cells form a map of each environment the subject has been in and responds accordingly in each environment. Question is how do they switch to represent a different environment when the subject moves to a new environment? It seems unlikely that a different set of place cells represent each environment the subject has ever encountered because it seems that would require too many place cells (and since the cell counts in the hippocampus is I presume relatively fixed, "free" place cells run out at some point), so it must be the same place cells "remembering" different response patterns to different environment and responding accordingly in different environments.
From the wikipedia page on place cells: "In a different environment, typically about half the place cells will still have place fields, but these will be in new places unrelated to their former locations."
Muller, R. U.; Kubie, J. L. (1987). "The effects of changes in the environment on the spatial firing of hippocampal complex-spike cells". The Journal of neuroscience : the official journal of the Society for Neuroscience.