Heres' an "easy" way:
1) update list definition
SUBSCRIPTION= OPEN,CONFIRM
2) note to all subscribers that they will have to resubscribe in the near future
3) DEL listname *@* PW=yourpswd
There are other methods too including the method that requires each subscriber to send to the LISTSERV (not list)
CONFIRM listname
There is a list definition keyword that turns on that requirement but I haven't used it in years since often times, subscribers would send it to the list (and it would get distributed) and some other subscriber would post to the list a question about the just-broadcasted CONFIRM, or would say back to the list, "yes, I got your message."
/P