Howdy,
I can’t answer how changes might be made when booking with Costco - I assume they act like a TA and you have to work through them(?)
But, I did crunch some numbers last month out of curiosity when Costco came up in the forum - I was able to do more better than their packages by crunching booking rooms through Undercover Tourist and buying reseller tickets. Costco deal was slightly better for the deluxe resorts, but still a reasonable amount of money more than buying yourself ala carte.
Not sure if the Costco deals change, so my numbers are not definitive - just done for a random week.
Here’s the forum thread: Costco info needed - #2 by PrincipalTinker