I know its best to upgrade step by step, version by version.
Im running WP MU 1.3.3 and willing to upgrade but i can't find download links to older versions in the download page.
Is clicking the Zip link after entering some of this tags: http://trac.mu.wordpress.org/browser/tags is enough?
(ej. http://trac.mu.wordpress.org/changeset/1406/tags/1.5.1?old_path=%2F&format=zip )
Should that work?