Some rambling comments below, I may come back in later and touch up on some specific points. But if anyone has questions about the theater industry in general or Amusement Park Theater specific I can answer since that is my current career and has been for the last 5 years.
As someone who work full time in the theater industry, and even worked at Busch Gardens in the recent past, I will say that everything mentioned above about the industry is correct. EVERYTHING!
Just like every other industry the theater industry has both good people and bad people. Yes some producers will come in and just strip a show for no reason. Or because they didn't like the lead's new hair style. Or whatever.
But at the same time you have the old style of Busch Gardens where performers and techs LOVED to work, and would return year after year.
As for the casting call, it is common for productions to keep casting even after their show already has a cast, or even after they are already in production. You never know when you will have a performer drop out suddenly or an injury that now you have to replace someone, quickly. I am not saying that is what is going on here but it MAY be.
As to whether current cast were offered roles and then had that role recendid, I don't know. But if they signed a contract for said show (and Busch Gardens does use contracts for performers) then re-casting is not as easy as just saying, "Sorry we don't want you anymore."
But from past dealings with Busch Gardens, and Amusement Park theater in general, I know they hold the contracts till the very last minute. So anyone who has been "let go" probably has not had a contract for Howl-O-Scream or Christmas Town yet so it's not a problem that they decided to go with different talent. I highly doubt that anyone has been offered a contract for either sets of shows yet, those probably won't go out for a couple of weeks yet.
Something I am seeing a lot is designers and producers are coming in and if they don't get the chance to do an entirely new show, they at least want to recast. So that they feel the show becomes theirs. I know there was a shakeup in the Entertainment department this season so I could easily see some new people deciding to try out others and see if they can, in their opinion, improve the show.
As for KK, haven't seen anything but the video. Hated it but I also think this is closer to what most Amusement Parks are going for. Not to the level of Cedar Point's Luminosity but I bet that's what they were going for with NO time and NO budget. I doubt it will come back next year but if it does expect it to be MUCH improved.