A post in another thread got me to look at the terms and conditions of the upgrade. This is what I found.
3. One benefit of your new Marriott Rewards Premier Plus account is an Anniversary Free Night Award e-certificate every year after your account anniversary, valid for a one night hotel stay at a property with a redemption level at or under 35,000 points, instead of the category 1-4 hotel e-certificate valued up to 20,000 points you received on your Marriott Rewards account. Please note that completing this product change may result in a change to the month you receive your annual Marriott Free Night Award e-certificate. Anniversary year means the year beginning with your account open date through the first statement date after your account open date anniversary, and the 12 monthly billing cycles after that each year. For the list of participating hotels where you can redeem the 35,000 points, log on to MarriottRewards.com/Use or call 1-800-321-7396. Please see your rewards brochure, which arrives within 7-10 days of your new card, for more details.
While we all know the answers from phone agents can be unreliable, I decided to give them a call. He did put me on hold to talk to a supervisor, for what that is worth.
In my case,
my annual fee will be billed in January, but if I upgrade now, I won't get the free night until May. This is specifically because a product change generates a new anniversary date at the time of the product change. So right now, my anniversary is when my annual fee is billed. But if I took the upgrade offer today, that anniversary date would change to today's date.
The fact that the terms and conditions above are worded they way they are backs up what the rep told me. I will probably wait until late in the year now before I decide whether to upgrade the card. I was planning on canceling when the annual fee was due. However, I also need more points for LTPP. So I will reevaluate the decision in November to judge how badly I need the 10K.