Just look to Age of Sigmar for how the 8th edition roll-out is likely to proceed. All current 40 Factions will get a "Dataslate" Compendium (or similar term) that will be available for free download as PDF documents on the day of edition release. For your reference all of the AoS ones can be found here: https://www.games-wo...of-Sigmar-Rules
The Compendium for the Space Wolves will have a single-page dataslate (warscroll) for all of the units that we currently have available, and that Citadel produces models for, which will definitely include the Thunderwolf Cavalry and the Wolf Lord on Thunderwolf. I doubt we'll get any new units on release day, but we aren't likely to lose any units either. The great thing about this method is that every Faction will be brought up to 8e readiness and we'll all be able to play games on day 1. Also, the designers can do dynamic updates to tweak unit rules, characteristics, and points costs at any point, without invalidating any books. Let's not forget that GW is in the business to sell models first and foremost. They make the games to facilitate the sale of the models. Thunderwolves are a good model kit that sells well and makes them money. Until the molds wear out, they aren't going anywhere.
The codices (battle tomes) will come later, and probably at the new pace of many per year. AoS has 17 battle tomes, and 5 grand alliance books out already, and the game has been available for less than two years. I expect every 40k Faction will have its codex pretty quickly compared to all previous editions. These new codices won't need to include any unit information, as all of that stuff is already available via the downloadable PDF compendiums. Instead, those can focus on fluff (mostly copy and paste from all previous versions of each codex), relics and artifacts, faction-specific warlord traits, and lots and lots of new formations and detachments. However, I wouldn't expect that the formations will be free anymore. In addition to the requirement to field the right units, you'll have to pay an additional points cost in overhead to gain the formation's benefits (no more free chicken, which will really, really help with game balance). We'll also see the painting guides, picture galleries, and probably some unique special missions and scenarios in the codices, as well.
All of that being said, we don't really know what the 8e rules will bring (other than what GW has already leaked), and how the meta and individual units will be affected, so I'd definitely hold off on new purchases and modeling for now. However, with the new design system, they'll be able to much more easily "correct the ship" if they make an over-performing or under-performing unit, which enables agile correction of internal and external army balance.