First, I am new to Spine and an engineer. I am not an animator/artist.

I have inherited a project with some spine animations/skeletons. They work great with spine 3.6 in Unity.
But we want to upgrade to Spine 3.8 and would like to use these old animations. When I upgrade the runtime to 3.8 I get a message saying that the skeletons need to be re-exported with 3.8. When I click on the skeleton file in Unity I get the same re-export warning. See attachment.
I've got the Spine editor. I open it with version 3.8 and imported one of the skeletons (Json file) it loads the skeleton fine but the images are all missing. I do not have access to the Spine project file that these assets were created with.
Any suggestions as to how I should proceed?

Thanks