I have segmented around 3-4 times so far, most recently at BLT 6/29-7/11. It never seems to go smoothly.
I think it all depends on how your reservation is in the computer and what dates print out on your KTTW card when you check-in. Everytime we check in, the CM takes a while and gets confused when they look at the reservation.
For example, if you check in on 6/29 and have 1 night of dining and the checkout date is 6/30, then yes you will have to go back down and get another card the next day because that card will lock you out of your room at 11am. However, everytime we have checked in, they card is always printed with the full reservation dates on it (6/29-7/11) even though we had any segments within that.
We have been locked out of our room the next morning after check in for the past 3 visits because the key locked us out after 11am. It seems that the only way it works correctly (at BLT), is to have a different card for every segment of the plan, but the card has to be printed that way. Unfortunately, that means many stops at the front desk with every change for a new card.
It really is a pain, but if you want to segment that's the way they have you do it at BLT.