I've never heard of Disney raising the dining plan prices for the current year once they've been announced, but I suppose anything is possible.
If you're going in October, but waiting until then to add dining, it has to be done no later than 3 days before your arrival and will need to be paid for at that time. If it's a room only reservation and you're adding dining it becomes a package and the terms of the package become effective (e.g.in most cases a ticket requirement, package payable in full immediately). Also, if you wait until the last minute, there's a change fee of $50 that isn't waived very often even if you're paying Disney more.
