Error opening XLS file

We just recently moved to Office 365.  I am in the process of moving user data files to OneDrive.  One user started using her files synced to local machine.  One particular XLS file she tried to open said the file name had changed or file deleted.  She was unable to open.  But other XLS files she could.  So I took the file from the original data source and I was able to open it.  So I saved it as an XLSX file and she can now open.  

Any idea what may of happened?  I have more users I need to move and wold like to try and figure this out before before I continue.