A sweet transition from Bootstrap 2.3 to Bootstrap 3 using a Simple Form custom form builder.

custom_form_builder.rb

# lib/custom_form_builder.rb
# See https://github.com/plataformatec/simple_form#custom-form-builder 
class CustomFormBuilder < SimpleForm::FormBuilder
  def input(attribute_name, options = {}, &block)

    if options[:wrapper_html]
      options[:wrapper_html].merge! class: 'form-group'
    else
      options[:wrapper_html] = { class: 'form-group' }
    end

    # inline checkboxes should not receive the .form-control class
    unless [:remember_me].include? attribute_name
      if options[:input_html]
        options[:input_html].merge! class: 'form-control'
      else
        options[:input_html] = { class: 'form-control' }
      end
    end

    if options[:hint_html]
      options[:error_html].merge! class: 'help-block'
    else
      options[:error_html] = { class: 'help-block' }
    end

    super
  end
end

new.html.haml

# app/views/devise/sessions/new.html.haml

# This example uses a standard Devise view to illustrate how the cstom_form_builder can be used
# to generate a Bootstrap 2.3 AND Bootstrap 3RC2 compatible markup.

.container
  .row
    .col-12.col-sm-12.col-lg-6.col-lg-offset-3
      %h1= t('titles.sign_in')

      # Please do notice a custom form builder is in use here!
      = custom_form_for(resource, as: resource_name, url: session_path(resource_name)) do |f|
        = f.error_notification

        .form-inputs
          = f.input :email, required: false, autofocus: true
          = f.input :password, required: false
          = f.input :remember_me, inline_label: t('activerecord.attributes.user.remember_me'), as: :boolean, label_html: { class: "visuallyhidden" } if devise_mapping.rememberable?

        # I also added .form-group here, but this is not critical and YMMV
        .form-actions.form-group
          = f.button :submit, t('actions.log_in'), class: "btn btn-default"

simple_form_extensions_helper.rb

# app/helpers/simple_form_extensions_helper.rb
# See https://github.com/plataformatec/simple_form#custom-form-builder
module SimpleFormExtensionsHelper

  def custom_form_for(object, *args, &block)
    options = args.extract_options!
    simple_form_for(object, *(args << options.merge(builder: CustomFormBuilder)), &block)
  end
end