I'm pretty sure I read in one of the 'weekly update' posts that bungie have been doing. That the engrams have been fixed so that you will get the same colour or above.
E.g a decoherent engram would get you a blue or anything higher.
Please correct me if I'm wrong here.
If that is correct, I have just had 7 blues decoded at the tower and at least 3 were green!
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When the patch goes live. So, engrams earned after Tuesday will decrypt into the appropriate level.