Hi,
the docs for upgrades all recommend doing an in place upgrade, as this is what is tested internally at Adobe. The alternative is to use CRX2OAK, as Feike suggested, which allows you to migrate a path from the source repository to the target repository. Please note though, the old and new repository must be stopped.
However, you should note that an in-place upgrade does more than just migrate the content, there are also post upgrade tasks which are run which transform the content for OAK, for example if you use MSM, then these tasks would be missed if you only did a content migration with CRX2OAK.
Regards,
Opkar