Of course that's step 1, you need to update the framework files ... What else did you expect it to be?
Look, none of us (the current maintainers) have had anything to do with CodeIgniter pre-2.0.3. I am the longest-serving current member of the project and my very first contribution was a bugfix that landed in 2.0.4 (and I did that as a random contributor, not as a project member of any kind).
If it wasn't all hosted on GitHub since 2.0.3, we wouldn't have downloads even for 2.0.x ... Prior to GitHub, all I know is they've used BitBucket for CVS, and the official downloads were on EllisLab's servers. Whether they didn't/couldn't migrate tags between BitBucket and GitHub, or if there were tags at all - I don't know.
Even if that wasn't the case though, you still couldn't blame anybody for this ... 1.7.x is a PHP4-era version tree. If you tried, you probably wouldn't be able to compile PHP 4 today either, so why would anybody make a conscious effort to preserve software that depends on it?
And on top of that, you're looking for such archives so that you can upgrade from a version that was supposedly in development at an unknown point between 7 and 9 years ago, but was never released. I mean, you were probably tasked with reviving a dead project, and I feel for you, but maintaining software up to date is a responsibility that's been neglected for the better part of a decade somewhere on your end - that's not on us.
That being said, you don't need literally every release since then - there's only 3 of them missing, all minor patch releases from the same version tree. Just download 2.0.3 and do all the steps from 2.0.0-2 together with it, it's not that many. Similarly, you can go the same way from 2.0.3 to 2.0.latest, then 2.1.0 to 2.1.latest, etc.
But given that 1.7.3 was never released, that may all be futile. It's an unknown
dev version - you may have changes within it that were later reverted or refactored, undocumented changes. If that's the case, nothing will help you, even if I were to provide you with all releases since 1.0.0.
