Yes, upgrading is only an add-on.
We address some of these questions in a more recent post and also the FAQ linked during the upgrade
2. Will I lose access to Internet Identity 1.0 after upgrading?
No. Upgrading adds a new discoverable passkey for Internet Identity 2.0 without removing your existing Internet Identity 1.0 authentication methods.
This means:
You can still log in to services using Internet Identity 1.0 as before.
You can also log in to Internet Identity 2.0-enabled services using your new passkey.