We are going to MK for 1 day (May 7th)and the Anna & Elsa option was not available when I booked all of our fp+ for the day to work with our Crystal Palace breakfast and CRT dinner reservations. Now, 2 days after the announcement, it is open only to very small groups. Like the poster above my only option was to cancel/rebook for 1 adult and 1 child and then retry to rebook the rest of our party of 8 for similar times and somehow that process made me nervous so we are my dh and I will stand in line to hold places for standby while the princesses and their family take in nearby rides. We only need the 2 princesses and their Moms to attend and get photos so this is our best option. I did call helpline and they said that it could be very difficult for large parties to make the a switch in segments.
As someone who plans 1 or 2 visits per year for family and friends (retired event planner) I think watching this process unfold should be VERY interesting. It may be good for crowd control and Disney, but it sure doesn't leave much open for spontaneous fun.