Don't waste the motes of light, the weapons aren't updated to the standard damage of the expansion. Meaning they are still only 300/300 and not 331/331 max.
[b]Edit[/b]: Tried with Timurs Lash, just changed the upgrades and not actually upgrade the total damage.
[b]Edit v2[/b]: With the coming House of Wolves expansion, you will be able upgrade ANY Legendary gear, so that means Iron Banner weapons as well.
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Edited by VanZanSlevin: 12/16/2014 10:11:58 PMSince Iron Banner totes itself as a test of your weapon and armor's power Bungie should definitely fix the reroll so that it ups the attack value on IB weapons. Even better would be if Eris had a reroll feature that worked on VoG weapons but only to up their attack since those really are meant to have the perks they have.