HI Bogdan

If you were working with minor increments of Tableau Server such as restoring v10.4.3 to v 10.4.1 you'd be OK, but it isn't possible to do this across major version changes, just as it isn't possible to publish from a v10.4 Desktop to a v10.1 server.  There are too many features introduced that aren't backward compatible.  THis article has more info - Workbook Version Compatibility

It might be with the recent changes in the v10.5 release that allows you to save back to an earlier version of Desktop (v10.2+ though), that this may well change with server backups.

Our approach when managing an upgrade is we take a full backup prior to upgrade and in the event we need to downgrade, we will restore the original backup.  The downside here is any content changes on the server post upgrade will be lost.  We try to keep the transtion window very small and set user expectations, discouraging much activity during this time.

Donna

Ditto to everything that Donna just said.

Ditto on the ditto