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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A slight clarification to the previous replies. The upgrade process is twofold: armor and weapons. In the case of weapons, the previous information given is correct. In the case of armor (which is what you are asking about), the previous responses are partially correct and not really spelled out in the FAQ. If you have an armor piece with a light level of 30 (pre-dlc), you can upgrade it to 36 and it will take on the stat roll displayed. If your armor piece is already light 36, you can still utilize the upgrade. However, all that will change is the stat (Int/Dis/Str). This is beneficial if you have an armor piece with a low stat roll and Xur is offering a higher roll. In both cases for armor, the upgrade tree is reset and will require you to re-level it.