I doubt that it will work that way with all rares. Im sure that there will be a select few that are tied in with the expansion that will do that but for the most part the rest will be trash like always.
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Since they are trash, not only you believe they are trash but the vast majority of the community does as well, wouldn't it only make sense to have this option that you don't believe will happen?
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I personally think there are enough ways to get legendary gear that something like that would not be necessary. It would be fun to do that with a few niche items but I could not see myself going through a bunch of hoops to upgrade every blue I drop.