The biggest cost of changing development partners is often the time a new team spends understanding undocumented decisions, architecture, deployment processes, and existing code.
Good documentation and ownership terms should be established from the beginning, even when there is no expectation of changing vendors.
A clean handoff should include the complete codebase, deployment information, architecture documentation, important technical decisions, and the access required to operate the product.
Ask development partners how they maintain documentation and handoff readiness before the project begins. A disciplined handoff process usually reflects a disciplined development process overall.