I can't give you a definitive answer, but I had a similiar situation recently which may apply to your query.

I recently changed the outbound on a paper ticket to a few days earlier, and was told that the ticket had to be reissued. When they reissued the ticket, the validity dates were changed to reflect the one year max stay based on the new departure date.

I think if you can arrange the date change without a reissue, you will probably get the extra days of validity. But I would guess that whoever is doing the reissue is well versed in updating validity restrictions for date changes.

Hope that helps