Changing Plugin/Device Configuration Fields

Requesting some advice here... is there anything special that one needs to watch out for when changing (adding/removing) configuration options for devices or plugins? I realize you have to be super careful about modifying any items that a user may be dependent upon for triggers or actions or what not.

I'm just wondering... say that I add a new boolean field with a default value to the Devices.xml file. When a user creates a new device, obviously that device will get the new associated property. But what about for devices that already exist -- how is the value/default value handled for those?

Thanks,
Adam