guidance coloring 1 shade lighter

I've been coloring my hair Med. Brown, but it is probably closer to lt. brown.  I do my roots every 2-3 weeks (very gray) with permanant color.  I recently ordered lt. brown wefts and want to color my whole head lt. brown.  I read in one of these posts that color can't lift color.  I don't want to bleach to lighten 1 shade...how does a person go about it?