Impacts on historical data table if user names are modified

Summary

What is the impacts on historical data if user names are modified

Content

Our company never had a set policy about the naming convention in creating user account name in Fusion. With multiple administrators, some were created with all lower cases, all upper cases, or the first letter is upper case and the rest are lower cases. 

Security Console allows changing user name.  However, we have concerns for the historical data.  If we update the user names, will all the data tables be automatically updated to the new user names?  If not, does that mean both old and new user names would appear in the search field and reports?  Please advise about the system design.