[development] Upgrading Drupal

I suspect I know the answer but am looking for reasons to convince a potential customer. The question is: Can you jump from Drupal 5 to Drupal 7 in one fell swoop? I see upgrading contribs as one of the biggest obstacles with API changes being the next one in line (assuming there is site-specific custom code).   Nancy Injustice anywhere is a threat to justice everywhere. -- Dr. Martin L. King, Jr.