@0x7C0, what applies to the Galaxy S7 SM-G930F (e-0.7-nougat) does not necessarily apply to the Galaxy S9 SM-G960F (e-0.8-oreo). But I am looking for clarification.
But as @Manoj speaks from his experience, it can be »If you are coming from a higher OS version it may have issues on some devices.« Last certainty is only given when the operating system change to /e/ OS ROM was successful.
This information → here the conclusion can be drawn that /e/ OS ROM e-0.7-oreo (S9 · SM-G960F) and StockROM Android 9-pie S9 (G960FXXS7CSJ3) are much better compatible than /e/ OS ROM e-0.7-nougat (S7 · SM-G930F) and StockROM Android 8-Oreo S7 (G930FXXS7ESL9).
These are the same requirements as for @ Fab2B. Accordingly, the chances are good that a successful OS change to /e/ OS e-0.7-o- dev (oreo) can be accomplished.