Replacing digitizer with same adhesive

if its a recent enough repair, you should be able to pull up the digitizer without removing the old adhesive. The adhesive sticks to the glass much easier than it does the frame. Remove the film, then reheat the digi and stick it down. Might want to give it some extra heat since that adhesive doesn't like being restuck.

If the adhesive tears or won't stick, then I would remove the old adhesive. Its not hard, it peels right off. Peel it all off and use red tape to stick it back down. Do not use anything metal or sharp on the old adhesive. The paint on the glass scratches very easily.