If you buy Maya you get the disk that contains all the versions, Win 32/64, Mac 32, Linux 64, full versions.
I think that you also get a years subscription to any upgrades from autodesks site, which you can download any version.
You can also transfer the licence from one machine to another, even if you change OS, you need to contact autodesk.
Not too sure about the trial though.
"No pressure, no diamonds" Thomas Carlyle