[support] multiple issues after downgrading...to export/migrate manualy?

Dear friends, In recent attempts to upgrade a fairly elaborate and content packed Drupal site (hosted at 101sitehosting and managed "mostly" through web interface of installatron) we failed to successfully upgrade from 5.x to 6.0 version of Drupal. As script gives option to downgrade the installation we tries to do so and site is again operational but quite a few content-type related issues appeared (articles, forums, book pages got mixed up).

What would be the good way to export existing content+users and import it into new 6.0 setup or maybe even to import it first to new 5.7 setup and then re-try the upgrade?

Thanks for any advice given!