You don't actually book a night with the voucher. What you actually do is book a night on points, with a note to the hotel that you will be using a voucher instead of points. That's why the scheme you proposed can't work; the voucher itself is not used at all until you check in, so I don't see how it could be extended by anything having to do with the reservation process itself.

The date of expiration is printed on the voucher, and you have to hand it in. (Even when it's an electronic voucher, it's only electronic to you, you still have to print it and hand in a printout of it when checking in.)

So the only possible way of extending a voucher would be for Best Western "central" to cancel your current voucher and issue a new one with a later expiration date. But I've never heard of them agreeing to such a thing.

So expect that you'll have to use the voucher by the expiration date, or else it'll go unused.

(Factoring out any not-possible attempts to extend the expiration of the voucher, it is flexible, in that you can keep changing your reservation, cancelling it and making a new one, and since the voucher is never in the reservation itself, all that doesn't matter. All the reservation sees about the voucher is a text comment to the hotel. So you have the same flexibility as when booking with points, except that you have to use the voucher by the expriration date.)

Btw, in case it's not obvious from the above, to be available for use with a voucher, a room has to be avialable for booking with points. Voucher room availability is identical to points room availability. (So, for example, you can't use a voucher to reserve "fancier" rooms that are not available on points.)