thanks for the replies, everyone! Sounds like I'm good to go!
As for the upgrade question: I'm guessing you could do that, based on this info from Mousesavers:
"Upgrading Tickets
You can apply the value of an unused ticket, or even a partially-used one with days left on it, toward the purchase of a higher-priced ticket. I strongly recommend doing this at a theme park Guest Services window, NOT at the Downtown Disney Guest Services office, which seems to have issues with doing upgrades properly!
If you haven't used the ticket at all, they'll apply the price printed on the ticket to any higher-priced ticket.
If you have used one or more days of the ticket, they'll apply the current gate price (which may be more than you paid) to any higher-priced ticket. For this reason, if you've purchased a discounted ticket it is best to use at least one day of your ticket before upgrading!
You must upgrade partially used tickets within 14 days of first use, even if you have the No Expiration option.
Tickets cannot be upgraded beyond 10 days total (i.e. if you purchase a 7-day ticket and want to add 3 days, that's fine, but you can't add 4 days). Once a ticket has been maxed out at 10 days of admission, the only upgrade possible is to an Annual Pass.
If upgrading partially used tickets with the Water Park Fun & More feature to an Annual Pass, you must purchase a Premium Annual Pass (which includes the water parks). "
But I don't know if there is some nuance or rule I'm missing....FWIW.