In my 8 years of teaching in Oakland, I was lucky to never have my grade level changed in the middle of the school year, but I had plenty of co-workers who did. The schools try to staff for the kids who will come but they don't actually know who is coming and in which grades. So every year, teachers would spend hundreds of dollars and countless days on getting their rooms ready, only to have to switch in mid-October.
Here's a very brief story about it and another teacher's thoughts, very well explained.
My Little Sister's teacher is being consolidated and she LOVES her teacher. I happen to know that the teacher she's getting is very very good but she's spent 6 weeks bonding with the teacher she has and starting over is scary.