No! Doing ProAddiction treatment to regrowth you don't need to redo the part that was previously done. You need to apply ProAddiction treatment only on the regrowth and leave it for the processing time. Before washing the treatment off emulsify it through the entire length and then rinse 90% off the entire hair length. When starting the heat process work only on the regrowth and touch up the hair that was previously done minimally.  For example: if you did 7 strokes with the flat iron the first treatment do 6 strokes with the iron on the regrowth only and the 7th one through the entire hair length to refresh the previously done section of the hair.