amcl first map only

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I saw on the amcl wiki page a new param, first_map_only param was added. I'm unsure how to use it exactly.

It appears like amcl can take in a new map on run time and use it. However, I'm not able to find another package that allows me to do that. It would be nice if my robot would be roaming around in the first map, and as it drives around, it can receive a new map(which has been updated), and continue to move around.

So here is what I'm curious about.

1. What is the param, first_map_only, intended to be used for?
2. If it is designed to be used as I described, is there any existing packages that allow me to do that? Or what should I modify to do what I'm looking for?

Note: I have looked into using gmapping to do dynamic mapping, but think I don't think I'd be able to use the actionlib to move the robot around because the initial position change every time gmapping starts.