color on color

I need to be educated about coloring...My hair is just below my shoulders and is a level 6 (My hairdresser has said that and that seems to be important information here so I thought I would include it.) Anyway, for a little over a year I have been getting it colored a reddish-brown that is slightly darker than my natural. I have not really found a shade of red that I like on me and want to go back to a warm, golden blonde. Before the red, my hair was highlighted every 3 months or so. I've been getting my hair colored since forever and I imagine this isn't going to be an easy process. My hair is probably in the best condition it has ever been..it is thin and apparently susceptable to damage. I say "apparently" because it is manageable and healthy looking to me, but the "inside" suffers with processing! My feeling on that is I really don't care what the inside is doing as long as it looks good. My question is how can I achieve a warm, golden blone color after I've been getting it colored reddish brown? My hair is do for a color (honestly, the red has faded quite a bit so I would say it is more of a light to medium brown right now) and I had a different person do my red than the blonde. The person who did my red doesn't feel comfortable with blonde, and suggested I go back to her to have this done. I want to know what is involved exactly with this process. Any help would be appreciated. Thanks