A 9-day base ticket (no options) is worth $415.35 gate value. A 10-day ticket is worth $426. At that point, you're at more $$$ than a Platinum pass - which is $797.69 for a non-FL resident, non-DVC member. Plus the APs can park hop, and you get free parking if you happen to stay offsite, and your Photopass downloads are included. I don't see any reason you wouldn't want it unless you aren't 100% sure you'll come in December.
It doesn't matter when you upgrade to the annual pass. Just don't wait until after the end of the last day on your 9-day ticket. Earlier would be better so you can take advantage of merchandise and dining discounts. Also, the "use it first" advice has nothing to do with price bridging. If the CM doing the upgrade does their job right, you'll get the correct price and expiration date no matter whether or not you use the ticket first. If they don't know what they're doing, they'll mess up one or the other (or both) even if you have used the ticket already.