I have tested the change of metadata on an existing test deposit:
https://inria.halpreprod.archives-ouvertes.fr/hal-02527543
(which is a SWHID scenario deposit)

• link to AffectationRO example: https://webapp.staging.swh.network/api/1/raw-extrinsic-metadata/swhid/swh:1:dir:79b8c8755dbed34f01a6a7184ffc196f3c58cb5d/?authority=deposit_client%20https://inria.halpreprod.archives-ouvertes.fr/
• see "discovery_date": "2022-02-14T15:31:37+00:00"

Does an update of an existing deposit get a new deposit-id?
Yes