"safely" downgrade - not really. It is a minor version, so it might be possible to export it, change the version in the data, and re-import it. Or possibly even just flip the version down in the database before connecting 6.4.0 to it. But if you do it, you won't be supported, and the effort might be a lot bigger than simply waiting for Zephyr to catch up (they'll be with you soon) or even testing the new version despite them not yet saying it's ok.
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