Though you can’t do this in a simple way such as using the Migrate option, for art assets like meshes you can manually export all of the meshes and textures and then manually re-create some blank materials with the same names and then copy and paste the nodes from the newer engine material to your new blank one and then just re-connect the pins. It’s a bit more work but provided the newer engine version hasn’t implemented new nodes that don’t exist in the old version, it should work. You’d need to create the root folder and import the textures, Then create the blank materials and copy and paste the material nodes from the new engine to the old one before finally importing the meshes themselves.
You should also be able to copy and paste all the actors from a newer engine map file to an older engine map as well in the same way, as it’s just storing references to the actors, and their positional data (transforms).
I’m in the process of doing this myself so I’ll let you know if it works.