Migration from version 1.7.2 to latest version

Hello, I appreciate it. Between versions 1.7.2 and 2.0.0, the following is stated:

All system files and directories, with the exception of the applications folder, should be replaced.

Put in their place of what, exactly? Do I have to migrate CodeIgniter versions one by one, even though I'm using the most recent release available?

Thanks

Kieran

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