Are you trying to go lighter? If not, you won't need to remove the old color, just start using the RC. If the old color is darker than your natural, then yes you could strip it with ColorFix.
I use Light Chestnut on my roots (trading off with Golden Chestnut) and it gets pretty dark. I can help you find a good color if you let me know which RC swatch best matches your natural color
that you're trying to get back to. As most haircolor goes darker on curlies, especially with processed hair, you will need to choose a color about 2-3 shades lighter to start out. You can always go darker if needed but it's better than getting too dark. And it often works out without any adjustment.
LMK if you'd like some help.