I think we should have a mechanism of changing the perks or roll of legendaries. I believe the best way to do it would be with a "fruit machine" aka " one armed gambit" etc, Vegas style. You have your current roll, you start playing and keep rolling each column until you have the roll you like. Each column has all possible selections , Bungie can calculate in the odds of getting a particular roll and that would translate to the chance of getting that roll. you can keep rolling as long as you have money to play.
I believe this would allow players to maintain weapons and armor they like and personalize/customize them to what they "love".
the ideal unit to use is evermore silver, something you buy, just like in real life. However for diligent players who have more time, there should be an option to also buy evermore silver with glimmer and or legendaries.
Exotics have glass needles, since they are exotics it should be harder to change perks.
suggest more options if you like this . If you don't like it explain why