Migration from version 1.7.2 to latest version

Replace your “system” folder with the "system" folder in  2.0.0.

> do I have to migrate version by version?

Yes, or it is good to check your app works in the next version.

However, since basically replacing the "system" folder is the same up to 3.1.13,
you could skip some instead of checking all versions one by one.

In any case, all other instructions must be performed surely for all versions.

ci-phpunit-test | CodeIgniter Testing Guide | CodeIgniter 3 to 4 Upgrade Helper