@Volker Busch Thank you for your post!
This link/article is pretty old but it’s mostly relevant for this conversation.
Azure Recovery Services Vault Management - performing a vault swap - Microsoft Tech Community
You should be able to register the DPM server to the new subscription, just download the vault credential file from the new subscription for the registration step. This changes the registration for the entire DPM server, not individual Protection Groups so all PGs would then point at the new subscription/vault.
Deleting the old vault would be an unrelated process. You would need to delete the old data and registration from the DPM server in order to allow the old vault to be deleted.
Hope this helps!
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