CodeIgniter is a very popular PHP framework. The current major release is CodeIgniter 4. All applications built on the CodeIgniter 3 framework or earlier should be migrated to CodeIgniter 4 (or other modern framework) as soon as possible as the support for the framework as well as the version of PHP that powers the framework is coming to an end. Once this support is gone security updates will only stack up and put your applications at risk.
MarqOne Technical Consulting LLC is experience with these kinds of migrations. While they are not trivial they are typically much less expensive than a full application re-write. It is also at this juncture that options for migrating to other platforms should be considered. The Laravel framework is the #1 PHP framework used at the moment and something worthwhile looking at as the ecosystem is much richer than the CodeIgniter eco system.