Some colourists are a little bit shy when performing colour changes. They often think that using a highlighting technique will give a better result, when in actual fact it may not be the best option. Gently lightening the base colour by one level and then colouring it with your chosen shade would to me be the simplest and best method of going a little lighter. Speak to a confident colourist and go armed with some photos of what you like and hopefully they will be able to assist you.