All the books (again, with the caveat that older books may not be optimal for the new core rules [but truthfully the core rules aren't changing wildly, like 5th to 6th edition 40k, really more of a tidied up 8th edition with a brand new setting]) are designed to be compatible.
Support will be towards the new book, new setting, however, with End Times being in the past. Viable, but not current.
Expect armies to have fewer units in their core books, which will be heavily focused on the fluff for where they are now, what they've been doing in the centuries immediately following the sundering. This will create a more "balanced" pool as they will be rapidly produced and released (consider a scale even somewhat faster than what we've had for 40k these past few years).
These will then be expanded on with supplements, not intended as money-grabs (as I am sure they will be received by the majority of your readers), but more as guided hands to acknowledge deficiencies in books, or even "global meta" changes. The first time that Games Workshop will be openly acknowledging things that need changes.
These units will typically get white dwarf rules treatments heralding the release in hardback of all of the new units from the previous month(s), for a new setting expansion, which will pit several of the races together (representing a collision).
As these expansions are not permanent in the world, so too will these models not be. They are intended to only get one template injection mold life-run, the same as the books will be printed only once in hardback, then delayed paperback.
They will of course remain legal throughout all of 9th, they will just be more limited eventually, the same as the end time models will not be available forever, the same as XYZ model is no longer available (just with a shorter life span than previous experienced). I mean... you can't get albion models anymore either, but that campaign was before people whined on the internet, so I guess that's why no one's complaining.