I worried about the same thing - we'd signed up to volunteer and picked out a day, but doofus me - I forgot to finish the sign-up process by clicking the link they sent me via email. When I saw GAD was ending, I hurried to get the sign-up finished and as a just-in-case, I emailed Disney's GAD team. Here's what they said to me via email:
Thank you for contacting us regarding the "Give A Day. Get A Disney
Day." program.
For those guests that are currently registered and committed to a
volunteer opportunity with the "Give A Day. Get A Disney Day." program,
a voucher will be held in reserve to be issued upon completion and
verification by the organization. Our records indicate that your family
is committed to "Meals on Wheels Delivery" with Meals
on Wheels, Inc. As long as this opportunity is
completed and verified, then you will be eligible to receive a voucher.
To sign-up for e-mail updates or for more information including a list
of frequently asked questions, you may visit
www.disneyparks.com and
click the link to the "Give A Day. Get A Disney Day." program.
Thank you for your interest in our "Give A Day. Get A Disney Day."
Program.
Sincerely,
Aaron Silver
"Give A Day. Get A Disney Day." Support Team
So, it looks like if you're signed up and confirmed for a specific day, you're good to go.