You can't just have them dropped from the reservation without cancelling and rebooking it, and if there is no availability for the new party size you have to give up the restaurant. The suggestion usually made is to make the reservation for 8, and then if you can't change the reservation to accommodate the new party size, just show up with six and say two couldn't make it. It's easier to go down two than try and add two to your reservation when you arrive.
If you've already made reservations for six I would recommend trying to change them to 8, but make sure there is availability before you cancel anything.
This could have repercussions at CRT or a dinner show, which are prepaid, and possibly at the restaurants that require a credit card hold, like Akershus or California Grill.