> You probably want to do this QUIETly...
>
> QUIET SET listname NOCONCEAL FOR *@*

On the other hand, you probably want to ask subscribers why
they're SET to CONCEAL. If you UNconceal them without asking them
first, or at the least, without giving them the opportunity to
UNSUBSCRIBE first (so their presence on the list in not
revealed), you'll likely create some serious animosity among
subscribers. To reSET them to NOCONCEAL "QUIET"ly would be even
more egregious than simply doing it without asking them first.
(Maybe you've already done all this "checking". If so, my advice
can be ignored.)

Peter R