1.) new users do not need to have defaults explicitly defined
2.) no script needed, unless you want one for other reasons
3.) stanzas for current users don't have to update (sorta)
The way /etc/security/user works is that any attribute NOT defined in a user's stanza is picked up from the default stanza.
So a default user should have a mostly empty stanza, possibly with nothing but an "admin" value.
When an attribute is changed for a particular user, for instance by using chuser, then the new attribute/value pair is written into their stanza and overrides the default stanza for them.
So look through your /etc/security/user file for stanzas containing any of the attributes who's defaults you've changed. Those will be the only users you'll need to change if you want them to have the default values. The safest way to change them is with chuser, but if you really know what you're doing you can edit the file directly (but back it up first!).
Admin Horror Note: On a machine I recently took over administration of, the /etc/security/user file contained oodles of ^M, which were causing strange behaviour when trying to do simple lsuser commands. Someone had ftp'd the file in binary mode to a Windows or DOS machine, edited it, and ftp'd it back in binary mode. <shudder>
Rod Knowlton
IBM Certified Advanced Technical Expert pSeries and AIX 5L
CompTIA Linux+
CompTIA Security+