A.V.
I would first find out your options. Then go from there. If there's not an option to have the preferred sitter in the AM, then just try to make it work for the next few weeks. Or you can ask the school they have any before school care program and then ask the program about slots. When our au pair was less than stellar, we put SD in the program at the school.
8 is like a mini adolescence. I'd sit down with the child and make her part of the solution. What does she do that the sitter has problems with and how can she make it better for the last few weeks? We had a sit-down or five with SD when she butted heads with her teacher in 4th grade. We told her that it was too late to swap classes for these minor run ins so how was she going to help herself over the last 5-6 weeks? She survived 4th grade.
If the sitter is often yelling, find out the circumstances. Have you just told the sitter to not yell or have you said x consequence for y behavior? Maybe she needs more direction?