After migrating 20+ legacy CodeIgniter apps from version 3 to version 4, we have a tight checklist. Start with a parallel install rather than in-place. Run both versions side by side until parity is reached. Use the upgrade tool only for routes and config — controllers and models almost always need a real rewrite. Plan for a deployment freeze of two days minimum. Test the auth flow last because session handling differs subtly. The detailed checklist follows…
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