But it seems to me that you don’t have to go back to stock Android to upgrade your Android /e/ OS version. (at least not for the vast majority of devices).
A reinstallation via recovery does the trick.
Going back to the original stock ROM is however necessary to downgrade…
As for the S9, a direct OTA upgrade will soon be available on /e/, so there won’t even be a need to reinstall the ROM via Recovery: