If your child changes school at some time other than in September, then schools should still support them with transition. A transition meeting between staff from both schools as well as you, your child (if appropriate), and other professionals working with your child would be a useful way to plan for transition.
Information such as strengths, areas of need and required support should be shared between schools. It may also be useful for your child to visit their new school before starting there so they can familiarise themselves with a new environment and new people.