There are two easy ways to do this. First, you can create an ID Repository that keeps the IDs and passwords — a kind of backup copy. The only caveat is that if the user’s name or OU changes, you must update the Repository ID.
Mind you, this does NOT work with an ID saved in the Person Doc in the NAB. Those IDs don’t update should the initial OU or name change — and keeping IDs in the NAB is not a good idea anyway.
Second, I believe that Notes 6.x (6.5 for sure, not sure about lower versions) has a password recovery function. You’d have to read up on the details — we don’t use it here.
Finally, you might want to look at WHY your users are losing their passwords so frequently. Our users hardly ever lose theirs, but we have only a password strength of about 5 (moderately difficult) and we do not expire the passwords. I’ve only changed my password on my Notes three times in 6 years, mostly because I was bored with the old one. If you are requiring frequent password changes in Notes, you might have to look at how much this is really helping your organization’s security vs. how much it is compromising productivity.