I’m not sure how to search to see if I can find out the answer so here goes…
I have both na iPad and an iPhone both syncing to my own WebDAV server for Omnifocus along with 2 macs, one running El Capitan and one Running Sierra. If I upgrade the iPad to iOS 11 and install the latest iOS version update that requires the new version will that mess up the sync for the iPhone which is on an older rev of the operating system?
I want to test the new rev 11 iOS and the new OF rev but don’t want to mess up the other systems in the process.
The version of OmniFocus 2 that is compatible with (and requires) iOS 11 is fully sync-compatible with all previous versions. However, it will prompt you to upgrade to the newest database format if it detects that all other devices being synced also support that format (see screenshot of Mac; the prompt on iOS is similar).
The only reason you would be prompted to migrate in this scenario is if you stopped syncing an older version and it eventually dropped off your sync history. If you do get the prompt, just choose “Later” and go find the device(s) that dropped off and sync.
You don’t say which versions of OF you’re using on the other devices, but if they are 2.x versions its possible that they are all compatible with the newest database format anyway. My recollection, though, is that you still use OmniFocus 1 which is not compatible.