Run this procedure to upgrade a 15.x multiplex database to 15.3.
IQ 15.3 introduces the concept of logical servers, which consist of one or more multiplex servers. The upgrade process changes the database configuration and modifies the login policies with appropriate logical server assignments to ensure that users continue to have access to same set of multiplex servers that they did prior to upgrade. Review the updated configuration and if needed, further modify it to suit your requirements.
See Sybase IQ 15 Login Policy Migration for details.