You can do a PT or DT but still watch your timing. Everyone's hair reacts differently to the ColorFix and you can't judge the porosity afterward. My cousin recently used CF to remove another brand of color, then re-colored with this other brand (she wants a lighter color so can't use the RC) and her hair did grab the color on the length quickly. We're actually going to use the CF again but just lifting a little with the shampoo method.
You may actually find that you do no have to recolor the length after the CF if you happen to lift to your desired color.
But if you do, I would do a strand test and keep your timing to 5-10 MAX.
Good luck! LMK how it comes out.
And be sure to just do the roots in the future so it doesn't get dark again. Only refresh the ends/length when it really looks faded, if it does.