I can now confirm that this process does indeed work to get art assets from a newer version of UE4 into an older one. I ran into some issues with opacity textures not being able to be exported out as tga files, but I worked around it with simply taking a screen shot of the texture in full screen, and then re-sizing to 1024x1024 instead of the 2048x2048 that they were originally. Took most of the day to re-create all of the materials and material instances and re-apply the textures to the meshes, but after I did that, a copy and paste of the map across engine versions worked as I suspected it would.
It would likely even work with blueprints provided, as I said before, the new engine version hasn’t introduced nodes that don’t exist in the old version.
I would also suggest that you try to get the engine versions as close to each other as possible. For example, the asset pack I converted supported up to 4.25 and my engine version was 4.22, but the oldest version the asset pack supported was 4.23, so I downloaded that engine and added the asset pack to it, before bringing over to 4.22. The closer you get the engine versions to each other, the less likely it is that there will be major changes to nodes etc. I actually did experience a few crashes when copy and pasting material node networks until I realized that it was because the “world aligned texture” nodes changed from 4.22 to 4.23. Copying everything but those nodes and then manually adding them back in fixed the crash.