You need to be precise. You apply high lift to the root area only, avoiding any previously lightened hair. Then go through it as if you were highlighting it with the Highlift, to pick out any darker areas and apply it on foils so you're not risking overlapping it on to the already bleached hair. Then I would process, rinse and tone down the mid lengths and ends to match the root colour. Prepare your client as Highlift tint will not be as cool or as clean bleach.