@Sascha For the ideal/normal scenario, the old PDS needs to be active & cooperating. It *may* be possible to get out in an "adversarial" scenario (PDS dead or gone rogue), but you need to have a recent backup and you need to have set an additional access key that you control up front while you still could (which is not a default).
There's a (technical) blog post about this here: https://www.da.vidbuchanan.co.uk/blog/adversarial-pds-migration.html