Answer
This happens when the Test database was refreshed from a copy of Production and still carries Production's TouchApps Server admin-info record — check the schema used for TouchApps Server admin info for a single leftover record in SERVICE_INSTANCE_TAB holding the old Production TouchApps Server details, and delete that record. Once it's removed, the installer correctly resets to genuinely blank default values, with the System ID field becoming fully editable so a distinct Test System ID can be entered.