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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It would have been nice to allow reforging our old IB weapons for new 331 specs, since the weapons are the same for all IB's. I managed to get the Scout and FR (bought), and MG (RNG) from past IBs, and really wasn't up for praying to RNG to lottery ticket me the new32 versions. 2 IBs = 1 RNG Blessing? No thanks. Reforging to 331 would have been a perfect opportunity for Bungie to throw the long-term players a bone. Ah well, I'll stick to regular Crucible this time around. No biggie. But this should be in the IB suggestion box for next time.