You cannot change a Pokemon's "overall potiential" or IVs. The only possible thing that happened is:
• You checked a different Chandlure, since it was a Lampent when you checked
• You misread what he told you about it's IVs.
EDIT: This is what it looks like when you have perfect IVs. Did the message look something like this?
>I see... I see... This Pokemon has outstanding potential overall. That's how I judge it, anyway. Incididdentally, I would say that its greatest potential lies in its HP. But its Attack stat is good, too. Hmm. And it's Defense stat is good, too. Although it's Sp. Atk stat is equally good. Its Sp. Def stat seems just as good, though. And, well, its Speed stat is good, too. Stats like those... They simply can't be beat! That's how I judge it.