After waiting several weeks, I finally upgraded my site from Joomla 1.7.3 to 2.5.1. At first I got errors and the update did not work. But after I turned off ftp in Global Configuration > Server > FTP Settings, the installation worked. There was a big list of database errors after the install ran, but I clicked on the new Database tab in the Extension Manager and clicked Fix and the database errors were fixed.
See this thread for more information: http://forum.joomla.org/viewtopic.php?f=625&t=691030