So on the gear front for current gear, first off keep in mind none of the current gear (e.g. from Tradu Mines or Shard Dungeons or Raids) other than capes is going to be in the game appearance-wise right at launch. So you’ll still have the gear equipped, still get benefit from the stats, etc., but it will look like basic gear from the new game. In addition, that old gear will not show up as transmog styles currently (again, because the actual look of the gear isn’t there).
At some point post-launch we will add that gear back in via a retro event of sorts. When we do that, the gear will start showing up as a transmog style again, and if you equip those old items because you hung onto them they will look correct.
So either way doesn’t really matter. If you have previously unlocked the transmog style by destroying it, you will still have it unlocked when the gear shows back up. If you don’t destroy it and hang onto it, you can actually use it as your “starting” gear when Reborn comes out for when you are leveling, and then stash it back in your chest.