A plugin for Chef::Knife which lets you move all nodes in one environment into another.
knife-bulkchangeenvironment is available on rubygems.org - if you have that source in your gemrc, you can simply use:
gem install knife-bulkchangeenvironment
Searches for all nodes in a particular environment and moves them to another
knife node bulk_change_environment "_default" development will move all nodes in the default environment (in quotes because of the underscore) into the development environment.
Please be careful when using this plugin, especially if you're already using environments with version constraints configured. You don't want all your nodes moved to the wrong one by mistake!