Let's not forget to say "open-source" instead of "free".  The English 
word "free" has too many meanings, and has historically proven one of 
Linux's biggest stumbling blocks for general audiences.

Being amusing and entertaining will go a long way.  Baiting proponents
of legacy operating systems (read Don Marti's "Linuxmanship" article
at http://electriclichen.com/people/dmarti/linuxmanship.html -- highly
recommended) is amusing up to but _not_ including the point where
it devolves to affirmations of OS-religion.  Be aware that at least
one dedicated flamer and Microsoft groupie is a regular Robert Austin
show attendee.

Consider referring to other OSes as "legacy" systems, per "Linuxmanship",
and reserve the term "upgrade" for installing Linux or changing versions
of Linux.