I have 2 pairs of floor-to-ceiling-length blackout curtains. They were originally a mid-blue, I dyed them dark brown a couple of years ago with machine Dylon, and now I want to dye them a kind of mustard yellow. Would it be possible to bleach them light enough to re-dye them? How do I do it? Will they fall apart (they're getting on for 20 years old but were good quality when bought)?