So really the only change is that you have the missiles make the decision to find a new target before hey reach the old location where they would have tried to do exactly that anyways, assuming it can find one when it does try. That's what I assumed when I saw the post about it, didn't consider being outside sensor range, or not having a sensor at the time though.
You mentioned "within the missiles sensor range", so an onboard sensor is required, since no sensor means even a range of zero doesn't quality implying there can never be any retargeting at all if there is no sensor on the missile, just as 7.1 when no sensor is present.
Two ways I think this could play out for a missile strike if there are no targets within the missile's own sensor range at the moment its original target, yet before it reaches the old position or its endurance limit, whichever is first. I'm guessing its B, but for clarity, I'll ask rather than assume.
A) If there is no other target detected by the missile when the current target dies, the missile simply terminate right then and there as it has no target.
or
B) It keeps looking while it flies to the old location, attacking the first thing it finds along the way, or shutting down only if it never finds anything when it gets there or runs out of endurance before it does?
or
C) Yep, a third option, something other than A or B happens?
In any event, I assume what the ship can see is irrelevant to whether or not the missile can see it with its own on board sensor,a nd if the missile can't see it, then no retargeting happens.