I have naturally ginger hair which is dyed dark chesnut brown, I want to dye it to a light auburn (Ashlee Simpson style) can I do this without having to strip the colour out of my hair??

You have a couple options. You can strip the color out (with things like color oops), bleach it, or if the color is pretty close, you can get deep cleansing shampoo, and leave it in for like 10 minutes every time you wash, and the color will slowly lift a shade or two. The down-side is they all damage your hair. If you're hair's not super fine, it shouldn't be too much of a problem. You just have to use treatments regularly for a while..

Hope I helped!