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.
The iBook Codex: Necrons states that the reduced version is Heavy 2, Blast. Doesn't that mean it can shoot twice?
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