Hmm. What are you putting it over? My experience was that when I put them over previously peroxided hair, "permanent" and "semi-permanent" tended to reverse roles. Permanent washed out immediately, and semi stayed forever.
ColorFix or ColorOops got them out, without much damage if any, but I wouldn't want to put myself through that unless necessary. They are also not supposed to work on semi, only on permanent, but they got the semi out for me regardless.
If you must do this I would MP a strand test first, then ColorFix/ColorOops it to see what it does.
But my real question, why not just henna it now?