> What I don't understand about this is how they're able to retroactively change the licence terms.
I think the concern is around provision 9 of the OGL: "9. Updating the License: Wizards or its designated Agents may publish updated versions of this License. You may use any authorized version of this License to copy, modify and distribute any Open Game Content originally distributed under any version of this License."
IANAL so I have no idea what all the implications of this actually are, or how a court might understand it. But at face value, the "any authorized version of this License" makes people think WotC would no longer authorize the 1.0 license and only authorize the 1.1 license, so further copying and distribution of material originally distributed under v1.0 would only be possible under the v1.1 license if it becomes the only "authorized" license.
Does contract law allow for such a thing? I don't know.
> Secondary question: does this apply to OSS as well? Can (e.g) Postgresql wait until the whole world is relying on their software, and then pull a licence change that makes it non-free/libre and requiring a paid licence fee?
No, because free software licenses don't tend to have that confusing "Updating the License" or any way to retroactively change the conditions that the OGL has.
OSS projects could always re-license (at least the code that the project has copyright ownership of) going forward, but that wouldn't affect copies you already got under the previous terms.
I think the concern is around provision 9 of the OGL: "9. Updating the License: Wizards or its designated Agents may publish updated versions of this License. You may use any authorized version of this License to copy, modify and distribute any Open Game Content originally distributed under any version of this License."
IANAL so I have no idea what all the implications of this actually are, or how a court might understand it. But at face value, the "any authorized version of this License" makes people think WotC would no longer authorize the 1.0 license and only authorize the 1.1 license, so further copying and distribution of material originally distributed under v1.0 would only be possible under the v1.1 license if it becomes the only "authorized" license.
Does contract law allow for such a thing? I don't know.
> Secondary question: does this apply to OSS as well? Can (e.g) Postgresql wait until the whole world is relying on their software, and then pull a licence change that makes it non-free/libre and requiring a paid licence fee?
No, because free software licenses don't tend to have that confusing "Updating the License" or any way to retroactively change the conditions that the OGL has.
OSS projects could always re-license (at least the code that the project has copyright ownership of) going forward, but that wouldn't affect copies you already got under the previous terms.