Migrate to new server

Hi all,

I have a WPMU install with about 50+ sites on. I need to move this to a new server. Is the only way to move the whole WPMU to ftp down the files/export the WMPU database; and then uploading the files/importing the database to the new server location?

Or is there an easy way such as a plugin?

Its important that I minimise downtime, so I want to have an action plan before I start doing the migration.

I should also say: All the the site are using an old version of WP/plugins/themes. These are being updated overtime (obviously because of security issues) so the migration needs to not break legacy stuff.

Any advice gratefully received! ??

Thanks