Converting the data is simply a matter of placing the data files in an existing MySQL installation, and then running mysqldump, so this isn't a problem. The Mambo-to-Joomla migration shouldn't be a problem either. All in all, this would take a maximum of four hours to complete, and the loss of data should be minimal to none (some database structure requirements have been implemented with Joomla over Mambo, but nothing crippling).