The 1st big update was originally said to be targeted for this month, then rumors said November -- November is now supposed to be unofficially confirmed. It's allegedly going to be delivered via Windows Update as a cumulative update, rather than the way Insiders have been getting new builds so far. It's said that you will not have to be current with win10 updates, & that if you download 10 after the update's released, you'll get the new version -- IOW it sounds like the old service packs.
Until it hits we probably won't know if the components for an ISO will be stored on the hard drive like the upgrade to 10, or the upgrade to a new build of 10. Likewise we won't know the extent of changes to the actual files that make up Windows. It may turn out that the way Insider builds have been updated is the preferred way to go, installing the new version, optionally migrating apps & settings, then cleaning up all the debris left behind.
Assuming that that stuff won't be known for a few days or maybe a week or so after the update is released, and that it may take that long for reports of big problems to surface, it might be best to prevent your system from updating right away when this big update is 1st offered. And that might mean a bit more diligence come November, e.g. checking Windows Update as soon as win10 fires up, watching for this update to hit so you can stop or delay it. It also wouldn't be a bad idea to really keep backups current in case you miss it, & win10 updates itself automatically.