Losing EPS Level Activity Codes when importing contractor .xer

Situation:  The contractor updates his schedule in his environment.  He sends the .xer to me and I update his previously imported .xer using the update existing feature in P6 in my database.

Problem:  Any EPS Level Activity Code I have put on the contractor's file in my database, before updaing the file, is deleted when I update the existing .xer.

How do I keep the EPS level codes that I placed on the file that is in my database before updating the file using the new .xer from the contractor's database?