To me, the bottom line is what's best for the member. Unless they're keeping ties with some members of the ward, it makes no sense for you to keep them. Since we know where they will be, it might make sense to move them to the new ward so they can welcome them when they arrive. But it would be nice to let them know of the situation so they don't go and try to visit a empty lot.
The alternative is to be unserved by any ward and to count on them presenting themselves when the time comes. But there could be some situations where that could be a problem. Their LDS Tools won't function and should they visit a temple and have a problem, there will be no bishop to call. And CHQ is likely to pester their family members as to their whereabouts.
I think the two handbook references shows that they should be a member of some ward and not cast into the multitude of the lost just because they can't attend. Granted, it will be an thorn in that ward's stats, but my ire is fanned by people who care more about their stats than the well-being of the members.
Now, if this was a problem with the system, I think we'd have heard more by now. Maybe the computer is trying to tell you something.

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