I’ve tried to migrate from owncloud 10.6.0 to nextcloud, but in all cases (migrator and manual installation) I got the error “Exception: Updates between multiple major versions and downgrades are unsupported.
Update failed”
What can I do?
Afaik only a migration from ownCloud 8.5 to 10.5 is currently supported. You might try to change the version ownCloud version number in your configuration to a 10.5 one, but as usualy you will do this on your own risk 
here it was said that a manual upgrade from owncloud 10.6 to nextcloud 21.rc2 worked without problems:
were there still big changes between rc2 and the release?
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I just did the migration a few days ago: here is the experience report: Migration from Owncloud 10.6 to Nextcloud 21
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