Yes it changes every single day because merchandise shipments arrive almost daily. Sometimes they can get new merchandise in later in the day rather than earlier, but when I talked to the CM's in the spring they said that didn't happen much.
I will tell you that I had a really hard time finding a silver castle this spring, and in October when I got my gold mickey (new design) there were people looking for the castles and they were gone. However, this fall people on the DIS have been reporting that they have had no problems finding the castle; or they only had to try a couple times to find the silver Castle. First trip I looked everywhere; this trip I only went to stores in the MK. I believe there is also a
Disneyland Castle Alex and Ani online, so if you want Cinderella's castle for WDW make sure you are ordering the correct one.
When I bought my bracelets, they were not listed on the
Disney Store website. They are listed now. I'm thinking that can be a positive and means they are addressing the supply demand. I couldn't find it on site despite multiple attempts at different locations, and parks merchandising was great about tracking one down for me.
If you don't find your castle on site and you can't catch it in stock at the Disney Store, call parks merchandising - I got mine in 1 call that way.
Now just have to find that Elsa dress for DD. You know, the one that me and apparently most of the US is looking for.
Good luck on finding your castle!!!
ETA: World of Disney is who told me to go ahead and get a hold of parks merchandising since they were having so much trouble getting the castles in stock. I am sure they will tell you the status on it - they know these are popular.