5. Plan migration around the complete website, not just its files
Bosseo describes an end-to-end migration covering files, databases, DNS and SSL, with testing before the DNS switch. Bosseo also says email records are mapped as part of the move. Those are material migration considerations because a website change can affect forms, redirects, certificates, domain resolution and mail routing even when the visible pages look unchanged.
Recommended approach
Request a written migration inventory for your current site. It should identify the website files and database, redirects, forms, analytics or measurement dependencies, SSL, DNS records and email records that need review. Agree on a staging test, a cutover decision, post-migration checks and a rollback or restoration discussion. Do not approve a move until you understand who controls each step.