Always backup your database prior to any upgrade - so even if you screw up somewhere you can easily revert back to a working state.
I'm not too sure but I believe you shoudn't loose your data when you upgrade to ez3. What I woudl do either way is to create a "clone" of your site on your localhost and experiment upgrading there.
There's no upgrading an entire 2.2.7 site to 3.0. We have a script for importing forums and users that we will release shortly, however since 3.0 is so different from 2.2.7 you should reconsider your site structure before importing.
I don't think it's possible to do it only by FTP. Unless you are able to set permission on directories trough FTP?