When they were talking about "upgraded" they meant with new perks and looks. Only some of the exotics got that treatment. I don't believe they meant that you couldn't infuse the other exotics to get them to max attack values.
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I certainly hope you are right. The limitation on exotics was surprising to say the least, so I hope there can be cohabitation between Year 1 and Year 2 exotics in the endgame.