You only fire the reduced version once, just like the stationary shot. So while you technically could elect to fire the reduced shot while stationary there is no benefit to do so. If you want to use the weaker shot, you should at least move to gain the Jink save and WS 1 if assaulted.
"Fetch me another plaything. This one seems to have broken." -Urien Rakarth
Space Marine- "This planet is ours, witch."
Eldar- "No ... This planet is theirs ..."