My understanding of this thread is that it is about
existing publishes, who have published material with a licence from WotC under the terms of the OGL can now "de-OGL-ify" that work.
And one aspect of that is their continuing contractual obligation not to use WotC's product identity. ...
A third is the IP rights (as opposed to contractual right) that WotC enjoys.
In my view these are not straightforward matters to unravel.
Particularly because, even if - as
@Matt Thomason suggests - these publishers move away from licensed works, their contractual obligations presumably remain binding (which is what everyone wanted, right - the OGL can't be revoked!).
Because Hasbro-WotC hasnt gone thru with breaching the OGL 1.0a (yet), I agree, this license remains obligatory.
The OGL 1.0a license only refers to the product, not to the publisher of the product. It is the product that must meet the terms of the OGL.
What is certain is, anyone else can continue to use and modify the OGL product under the terms of the OGL.
(Note. Switching from the OGL 1.0a to the new CC-BY 4.0 seems not a transfer. What happens is, the product has two licenses. Other users of the product can choose which license to use. If choosing, the benefit of the OGL is the ability to declare ones own Product Identity. The benefit of the CC is to draw inspiration from other Hasbro-WotC products under the terms of copyright rather than under the terms of the stricter OGL Product Identity agreement. I prefer the terms of the OGL, but unfortunately its author Hasbro-WotC has shown bad faith toward it. I view Hasbro-WotC offering the SRD to the CC as a constructive gesture. But it is a less ideal solution for an independent publisher, whether professional or amateur.)
I expect the ORC license will be excellent.
I see less difficulty with products that were released under the OGL 1.0a. If they were faithful to its terms, the fact their product AVOIDED Hasbro-WotC Product Identity, will protect them now when using a new license for the product, since there will be fewer copyright entanglements.
The question is, how dependent on the SRD is the product, really.
Now. To reverse-engineer the SRD solves most problems.
This new game rules system focuses on: public domain "names", abstract "concepts", and a clean "game rules" system.
Perhaps we call this new game rules system the Open RPG Engine: ORE.
Most of the time, it is possible to update an OGL product to the new ORC license and instead use the new ORE rules. This normally requires rewriting the product, but with little or no information loss.
I cant think of a case where a product cant update. Most of the changes would be trivial, like using the term underworld rather than shadowfell, or cambion rather than tiefling. Maybe the dragonborn are distinctive enough to need to doublecheck, essentially a dragon head on a human body. I am unsure an Egypt-esque crocodile head or a Norse-esque draconic snake with arms, or even a humansize dragon, would satisfy a dragonborn fan. Even so, these approximate equivalents have appeal in their own right.
Most of the time, to update an OGL product for the ORC license using the ORE system is a good solution.
In this solution, the old OGL product remains in place. But the new ORC update is carefully free from the Hasbro-WotC SRD and no longer derivative of it.
(The situation resembles publishing the same product for two different systems, such as one version for Pathfinder 2 and an other version for D&D 5e. The ORC update means there is one version using the OGL such as for 5e and an other version using the ORC for ORE.)
Another is their contractual acknowledgement of WotC's copyright in the SRD, which is also an acknowledgement that their own work is in some sense derivative of WotC's.
Many products that have zero connection to Hasbro-WotC products, used the license anyway because the license itself was a useful license. The use of the license in no way implies that a work is derivative. It only is a statement of agreeing to avoid Hasbro-WotC Product Identity, which is normally a trivial compliance, when the product is nonderivative in the first place.