As I wrote in the other thread https://moodle.org/mod/forum/discuss.php?d=229668&parent=1024132 I doubt a one step site upgrade 1.9 -> 2.3 is the right thing.
@documenters
Unfortunately the word "legacy" has a negative connotation, at least in IT. If you could invent an euphemism for it, the people will live happy forever. I propose "catacombe" or even "legendary (file) archive".