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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This is what people are talking about when they say Bungie is tone deaf, and slaps dedicated fans in the face. People who got the IB stuff last time had to grind everyday for a week, and I got the Lash on last day after about 120+ rounds. Spent a TON of shards and energy to upgrade them, and then find out later that the new gear is higher attack, and light level. I NEVER would have even bothered spending the ascendant material or even doing the IB at all if I knew that two weeks later that stuff was pointless. I HATE the way this stuff is being handled.