How to use old Debian repository for unmaintained Debian versions
Installing old Debian to prepare a migration is quite good when repositories are still available. But for old unmaintained versions of Debian, we need to change some things.
Hopefully Debian still keep all the old versions in an archive repository that is accessible by changing the default url. Even if during install it fails, you can set it up afterward.
I’ve wrote a little article on it, follow the link.