Cleaning up Inactive Usernames

 I am currently auditing our user id's in CFS, We have come across multiple Ids for individuals due to name changes.  We also run into the issue of trying to submit an ID to a user and having it already 'in use' by a termed employee from several years prior.  We are trying to move to a single sign on system in our business, with that our CFS user id's need to be the same as our system log in Id's.  What is the long term impact if we delete a no longer active user id from the system so