As per the title, I'd like to run an update on the offline index for testing purposes, but don't want it to swap to live view after completing.
Then I'd be able to test the crawling without the risk of affecting live searches.
The closest thing I can do in the Admin interface is to run the update, then as soon as it completes, quickly go to Start Advanced Update and swap live and offline view back again, and hope that nobody noticed!
