Updating CodeIgniter 1.7.3 to CodeIgniter 3.1.7

Thank you for your post. I am a little confused about something though. If I am to follow the upgrading guide how does one get a copy of the previous "system" folders for the earlier versions of CI as outlined in the Upgrade Guide as per the

"Step 1: Update your CodeIgniter files.
Replace all files and directories in your “system” folder except your application folder."

So, without access to the next version from 1.7.3 to version say 2.0.0's "system"/(kernel) dir how is anyone supposed to even know if their changes were done correctly? Not only that-As I mentioned above, its your guide's #1 step to upgrading in the 3.1.7 User's Guide!

Don't you see how that can be confusing to mention replacing something there is no KNOWN repository linked to? YET you are supposed to do that Step 1"?

Please explain... Thank you again!