Sorry for the double posting earlier - a little browser trouble.
Not quite sure I understand your last posting, but I'll try:
This won't hurt your existing users and their mail settings. If you already had some set with long user accounts then things will still work as expected. If you're running 4.0.1, and you do the upgrade to 4.1 the special pseudonyms will automatically get generated for you for already existant users, and then any additional ones added later will also get the feature.
In terms of setting up your mail clients, you always set them up so that the user account used to get the mail from the server is the full account - not the pseudonyms (those are just extra perks which happen to forward the mail to the user account). All users still use the user account for logging in, checking mail, accessing shares, etc. Its just with pseudonyms, you get the ability to have mail sent to .@yourdomain.xxx as well as the other features.
So you will lose any pattern you were trying to setup with all useraccounts (i.e. all accounts are as some will fall outside the range).
Hope that helps.
Jay