I think you should be able to get the etheric light back. When you dismantle an exotic item you get the exotic shard. I can only imagine that a lot of players will use their etheric light on something, and then wish they had used it on something else. I hope it's just a matter of time until we see this in a patch.
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If you used another shard to get it to 36 light, you still only got one shard. Now, if I use yet another to get to 42, I bet I still only get that original one. Once used, exotic shards and etheric light are gone forever. Ascend carefully, guardians. Also, complain until they change this to where you do get your major upgrade materials back. Also, old gear that you put 18 shards into ought to give more than 3 when you dismantle it. Maybe 8-10. Getting half back seems reasonable.