Thanks for all of the insight -- armed with this info, I contacted Costco (who proceeded to conference call in Disney). All the advice here was right on (and ultimately I was unable to change anything and will have to do it when we board). Here's a summary for anyone else in the same scenario:
1. You cannot move all the guests out of a stateroom or it is considered a cancellation.
2. One of my swaps was to put a 20 year old with other teenagers, and as noted above, this also isn't allowed until onboard.
3. I am moving around due to the connecting staterooms (i.e. which families will be connected to share bathrooms), just to answer the question by
@Girl of the South.
4. I think we will be able to get it all resolved when we board, just will take a bit of time at customer service.
Lesson learned -- before finalizing with Costco (or even Disney), make sure to vet the room assignments ahead of time.