To downgrade to High Sierra without erasing your drive is quite easy as long as you are using APFS.
1. If you have converted your drive to APFS, simply create a new volume in the same container as your boot drive. Do not set any size restrictions on that new volume.
2. Install High Sierra on that new volume according to this thread.
3. Boot it up and copy the files you need from the old volume to the new volume. The nice thing with the APFS file system is that the volumes dynamically share the same free space within the container. When finished simply delete the old drive.
I just did that and managed to downgrade to High Sierra without erasing anything. You of course need some free space on your old drive so that you have some space to work with.
The new volume should be the target, in the same container as your old boot volume.Can I just run the HS installer from Mojave with the new container as the target?
**EDIT: Nope. Mojave gets weirded out by it being older.