I completed The Witcher 3 on "Blood and Broken Bones" difficulty and want to try for "Death March" using my current gear (thankfully NG+ came along). Am I still eligible for the trophy for completing the game on "Death March" even though I'm carrying over gear from a lower difficulty playthrough?
According to this post by CDPR Community Manager Marcin Momont:
You can obtain the Death Match achievement when playing the NG+ mode.
I'm confident, based on context, that he means the Death March achievement by this, and so you are eligible.