Context: the new iOS enforcement of Privacy manifests is slated for In Spring 2024 Apple will require any app with a dependency on one of their determined SDKs to provide a privacy manifest file showing how they utilize relevant data types and APIs.
Question: As a developer with a dependency on one of these SDKs, do I need to update my existing app with a privacy manifest if it already includes one of these SDKs, or will I only be subject to add a privacy manifest for my existing app if I newly added one of these listed SDKs?
As how i understand your question and apple's privacy update it is mandatory to add a privacy manifest to your app, existing or new ones, when they get updated. Quote:
When you upload a new app or app update to App Store Connect that uses an API (including from third-party SDKs) that requires a reason, you’ll receive a notice if you haven’t provided an approved reason in your app’s privacy manifest
Further information: https://developer.apple.com/documentation/bundleresources/privacy_manifest_files/describing_use_of_required_reason_api