FYI, more generally - you never edit a persistent schema directly.
You must create a copy of it, apply your modifications, and then use DgnECManager::UpdateSchema() - with the caveat that unless you take care of remapping existing ECInstances to match the new schema, any existing ECInstances will remain unmodified (and potentially unreadable, if e.g. you deleted their ECClass).
ItemTypeLibrary takes care of all of this for you.
You must create a copy of it, apply your modifications, and then use DgnECManager::UpdateSchema() - with the caveat that unless you take care of remapping existing ECInstances to match the new schema, any existing ECInstances will remain unmodified (and potentially unreadable, if e.g. you deleted their ECClass).
ItemTypeLibrary takes care of all of this for you.