I have now tried to upgrade the Exotic Armour "Armamentarium" twice thru Xur and both attempts have failed. All materials relating to the upgrade (Glimmer, Exotic shard) are consumed but I do not get an upgraded Armamentarium out of the action and the attempt to upgrade the armour resets my progress.
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Upgrading an exotic via Xur is used to upgrade pre dlc exotics which had a maximum attack rating of 300 or light rating of 30 to the tier 2 Version which can be maxed out at 331 Attack rating or 36 light. Upgrading an Exotic via Xur clears it's upgrade tree progress. It won't affect stats, it will also not max out the upgrade tree. If you had a post DLC exotic, you may have wasted your shard and glimmer. [url=https://www.bungie.net/en/Help/Article/12457][b]Exotic Upgrade FAQ[/b][/url]