As for now, our idea is to make migrating a no-brainer. We don’t have worked out all the nifty details (as some of those rely on changes in ownCloud core that could be reverted), but we have an overall idea.
For the usual ownCloud administrator migrating will probably be as easy as uploading a single file, adjusting a single line of config or enabling a custom app. We aim to provide migration paths starting from ownCloud 8.0 up to ownCloud 9.1.
See also How will I be able to upgrade to Nextcloud from ownCloud? 