If an instance is customized so that a base-system field with a choice list on an extended table is promoted to the base table, or removed and a matching field on base table created, not having a Customer Update [sys_update_xml] record means that future upgrades will not take into account Customer Update records of the new field on the base table and will write the base system choice list, thus removing the customized list.
The most common example of the issue is with the Category field which is defined by default on Incident and Change Request tables. Often, the Category field is necessary on a Task table. Promoting the Category field from Incident or Change Request tables (or removing it and creating a new one on the Task table) means that future customization of the choice list will go into the sys_choice_task_category'Customer Update. If there are no Customer Update records for sys_choice_incident_category or sys_choice_change_request_category, future upgrades may overwrite the customized choice list with default base system values.
Steps to Reproduce
Create a customized choice list on a field.
Update the instance to a new family.
Return to the customized field.
Note that instead of the customized choice list being available on the field, the default base system choice list is visible
Contact Customer Support to confirm whether this issue applies to your instance. If so, they will discuss a workaround
Related Problem: PRB645540