To delete a user and add a new one or keep them both

To delete a user and add a new one or keep them both

A sales consultant has left the business and a new one has come on board. We would like to keep all of the old reps leads and potentials on the system categorised as theirs. We would also like to add the new rep to our users - how do we go about this? Is it best to change the old rep's user to the new one's name or create a new user altogether and delete the previous user?