migrate to actiontext (untested)

migrate_fields_to_action_text.rb

class MigrateFieldsToActionText < ActiveRecord::Migration[5.2]
  class ActionTextRichText < ActiveRecord::Base
    belongs_to :record, polymorphic: true
  end
  TARGETS = {
    ModelName => [:field],
    OtherModelName => %i[field1 field2 field3]
  }.freeze

  def up
    TARGETS.each do |model, fields|
      fields.each do |field|
        model.where.not(field => nil).each do |record|
          ActionTextRichText.where(
            record: record,
            name: first
          ).first_or_create(
            body: model.send(field)
          )
        end
        remove_column model.table_name.to_sym, field.to_sym
      end
    end
  end

  def down
    TARGETS.each do |model, fields|
      fields.each do |field|
        add_column model.table_name.to_sym, :tmp, :text
        ActionTextRichText.where(record_type: model.name, name: field).each do |entry|
          entry.record.update_column(field => entry.body)
          entry.destroy
        end
      end
    end
  end
end