we recently upgraded our TEST ILMT server from OS RHEL 6 to RHEL 7. Initially everything came up fine for one day, after that i got a error on my UI saying something went wrong. when we ran pre-upgrade report on OS checklist , i saw this on the report about db2 In Red Hat Enterprise Linux 6, the range reserved for system account IDs is 0 - 499, while in Red Hat Enterprise Linux 7 it is 0 - 999. This difference might cause troubles during the upgrade. In addition, the range 0 - 199 is prohibited from the use without a static ID reservation in the setup package. The IDs that are in this range might be reserved and used later by a package, and using them might cause the malfunction of the package. The following problems were found on your system: System group "XXXXXX" uses ID 101 without reservation. This ID is prohibited from the use and might cause migration problems. System group "XXXXX" uses ID 102 without reservation. This ID is prohibited from the use and might cause migration problems. Your system contains UIDs or GIDs in the range 500 - 999. Therefore, the Red Hat Enterprise Linux 6 defaults (system accounts limit on ID 499) will be kept to prevent mixing up the system and user accounts. If you can migrate your user accounts above 1000 easily, do so, and adjust the /etc/login.defs file to the values used in Red Hat Enterprise Linux 7.

Please note that in place upgrade from RHEL 6 to RHEL 7 is not supported. RHEL does not really support upgrade from RHEL 6 to RHEL 7, so expect troubles. We recommend and support migration procedure.

https://www.ibm.com/docs/en/SS8JFY_9.2.0/com.ibm.lmt.doc/Inventory/probdet/c_migration_between_computers.html