This happens from time to time. You are impressed by a principal or kindergarten teacher or PTA and are thrilled to get a seat in the school and then you find out that that person is retiring or moving on. What does that mean for the program? You should never pick a school because of any one person, or just by the test scores, or because of a program or grant. There are many factors that should go into your determination of a good school for your child. Stats famously fluctuating, budgets are cut, grants run out and staff (no matter how much you feel that you depend on them) have their own lives.
A good school, like any good workplace, will have a solid leader, the principal, who is the educational and fiscal leader of the school. They should also be a terrific communicator who will support their staff while attending to the needs of the families AND encouraging parents to engage and support the school. I would like to make the case that a good boss is also a good delegater. There may be coaches in the school who specialize in math or literacy who can support the teachers, vice principals who may be good with the budget or talented grant writers. The teachers should be encouraged to search for more effective ways to bring learning to their classrooms and they may be spear heading projects. It could be possible that there are great dictators who do everything themselves, and if that is the case in a school, it would be a disaster if there was a regime change, but my guess is that a good school is so much more than just one person.
A good principal will take pride in developing or maintaining a great school. If a principal leaves, after all that hard work (and it is an incredibly challenging job), they are going to leave the school in the hands of a trusted custodian of their legacy. There may be a vice principal within the school or a colleague from another school who is tapped to apply for the job. When a principal leaves there will be an interim acting principal until the school can go through the C30 hiring process. The interim acting principal may or may not want the permanent job. It is not automatic that the vice principal from within a school becomes the principal. The C30 committee is made up of teachers and parents and administrators from the school. The job is publicly advertised and the C30 committee interviews and votes on the applicants. If the school community is in accord this is a pretty smooth hiring process.
There will always be an adjustment period with a new leader. Some long term teachers may take the opportunity to retire. The principal may make some staffing and procedural changes. Many very well regarded neighborhood schools have weathered many administrative changes and have grown in strength and popularity and there are times when a new principal placement can breathe new life into a program.