Desktop integration is one of the trickiest areas for Linux migration. Rather than pitch the idea of migrating the whole department, you should first pitch a pilot migration with a few volunteers. Select users that are most likely to be able to handle simple support issues by themselves in order to minimize the impact on support teams.
In fact, some organizations (mostly tech-savvy companies) have allowed Linux on desktops as an official unsupported option. Engineers are usually the ones who volunteer for this, but don't overlook eager employees with less technical aptitude, who may be more likely to help their colleagues later. The result: management sees that integration is possible and, perhaps, less painful than the expected.
Several organizations I've worked with then appoint these pioneers as "Linux desktop ambassadors" to assist other users with a larger scale migration. A pilot program can help management feel comfortable with a larger migration, and you have a chance to build Linux skills -- in both users and support staff -- slowly.
This was first published in January 2006