update_scalars.rb

require 'psych'

def tree o, options = {}
  visitor = Psych::Visitors::YAMLTree.create options
  visitor << o
  visitor.tree
end

# create a YAML AST from a Ruby object
my_object = [ "(( foo ))" ]
t = tree my_object

# find scalar nodes and modify them
t.grep(Psych::Nodes::Scalar).each do |node|
  node.plain  = true
  node.quoted = false
  node.style  = Psych::Nodes::Scalar::PLAIN
end

puts t.yaml

# To figure out what the values *should* be, I usually parse the YAML to an
# AST and look at the values there:

p Psych.parse "(( foo ))"

Thanks @tenderlove!