Quick and dirty use of the wonderful xcodeproj gem to modify the CODE_SIGN_IDENTITY and PROVISIONING_PROFILE property of all configurations of each target where that property is set.

gistfile1.rb

#!/usr/bin/ruby

# Use the xcodeproj gem to modify code sign identity in all targets where
# that attribute is set.
#
# Kind of getto.


require 'rubygems'
require 'xcodeproj'

path            = 'Foo.xcodeproj'
identity        = 'iPhone Developer: Friendly Ghost (XYZ123ABC)'
profile         = '00000000-FEED-DADA-ICED-C0FFEE000000'
csi_key         = 'CODE_SIGN_IDENTITY'
csi_key_iphone  = "#{csi_key}[sdk=iphoneos*]" 
profile_key     = 'PROVISIONING_PROFILE'
profile_key_ios = "#{profile_key}[sdk=iphoneos*]"
project         = Xcodeproj::Project.new(path)

project.targets.each do |target|
  target.build_configurations.each do |conf|
    conf.build_settings[csi_key] = identity unless conf.build_settings[csi_key] == nil
    conf.build_settings[csi_key_iphone] = identity unless conf.build_settings[csi_key_iphone] == nil
    conf.build_settings[profile_key] = profile unless conf.build_settings[profile_key] == nil
    conf.build_settings[profile_key_ios] = profile unless conf.build_settings[profile_key_ios] == nil
  end
end

project.save_as(path)