Would it not just be better that the welcome PM is recreated if a reply is made. On a busy board, you would not really want them just closed, deleted it better.
Either that or make them non-replyable (better option), but contain a link to start a new PM if the recipient needs assistance. This link could then go to either a form, a new PM or even a help-desk, depending upon what the site uses as its chosen method of assistance. Some large sites use a full blown help-desk, so a link to this would be the best option.
I guess the best option is to have several choices and leave it up to the admin to decide which to go for. Every site is different, depending upon the numbers of new members.
