If we provide this kind of provision then there are multiple issues.
1. User has used default setting and according to that data is logged under server.log file.
2. Now after sometime user has changed logging format setting to something else.
So if you open the server.log file there are set of the records with different format. Some records with default format and some with new format.
Now when user uses admin console to view log records using 'Log Viewer', it will fail there because of different log records format in the same server.log file. Parsing of the record is failing there. Back end code is not aware of the old format and tried to format old log records with new format setting and fails there.
Also in 'Log Viewer' screen there are advanced search options to filter log records. If user had changed the log record format completely then it might also fail there.
Need your opinion on the same.