The target is out of range as soon as they are beyond 5 squares
The language is precise and unambiguous.
First, what a sustain entry means is that the duration of a power can be extended by taking a certain kind of action, and the sustain line will include any modifications to the effect that occur when the action is taken. Witch Bolt's sustain entry is a Standard action, but it has the proviso that it cannot be sustained “once the target is out of range”.
That could be thought of as ambiguous, but it really isn't: once's literal meaning is to indicate a one-time change. Another way to write the exact same meaning with different words is “If the target has been out of range…” The alternative interpretation — that it could be sustained if you get back into range — is unsupportable because it would require a different wording on the sustain action's conditions: “If the target is out of range, you can't sustain this power,” or “When the target is out of range…”
Since it doesn't say “if” or “when”, it doesn't work as if it said “if” or “when”. Instead it says “once”, which means that as soon as the condition occurs, the “can't sustain” proviso activates (and stays active, because there is no provision for ending it for that casting).