downcase! method not working on lower case strings?

gistfile1.rb

movies = {
    :H2G2 => 3.5,
    :AlienI => 3.5,
    :Minority_Report => 3,
    :After_Earth => 1.5
}

puts "What would you like to do (add, update, display or delete)?"
choice = gets.chomp.downcase!

case choice
when "add"
    puts "Added!"
when "update"
    puts "Updated!"
when "display"
    puts "Displayed!"
when "delete"
    puts "Deleted!"
else
    puts "Invalid command"
end


# second example

movies = {
    :H2G2 => 3.5,
    :AlienI => 3.5,
    :Minority_Report => 3,
    :After_Earth => 1.5
}

puts "What would you like to do (add, update, display or delete)?"
choice = gets.chomp

case choice.downcase!
when "add"
    puts "Added!"
when "update"
    puts "Updated!"
when "display"
    puts "Displayed!"
when "delete"
    puts "Deleted!"
else
    puts "Invalid command"
end