Audit before you design
Record current rankings, top-performing pages and existing URLs. Without a baseline you cannot tell whether the new site improved or damaged performance.
Plan redirects properly
Every changed URL needs a one-to-one redirect to the closest equivalent page. Bulk redirecting everything to the homepage is the single most common cause of post-launch traffic collapse.
Test before launch
Check metadata, structured data, page speed, accessibility and tracking on staging. Launch day is not the time to discover analytics was never installed.