I had a similar situation on our last trip. I thought we'd have 8 people, but two backed out after we had made the reservations. I called Disney and was told to leave the reservations for 8; there was no penalty for showing up with fewer guests.
The only reservation I changed was for HDDR, which was paid at booking.
It all worked out. My sister-in-law and niece ended up joining us, so the reservations for 8 came in handy.