To check the username format you are using:

1.) Start from your Core persona.
2.) Navigate to Settings > Security > User Name Format.

The Default Format is: {prefix} {firstname} "{nickname}" {middlename} ({maidenname}) {lastname} {suffix} {gradabbr}.

If you see
 {gradabbr}, in either the Default Format, or in the Custom Format, this means the Abbreviated Grad Year ('06) is being used.

If you have this set, however you are seeing the grad year as a single digit (ex: '6) - then there may have been a Data Refresh done where the grad year was entered as abbreviated to begin with.

To check your Data Refresh files:
1.) Start from your Core persona. 
2.) Navigate to Settings > Data > Data Refresh.
3.) Locate the Routine, and click Job History.
4.) View the processed file for the job.