as i wrote in another thread as far as i remember 2.02 is beta only becaus of some missing docs....
There's not much upgrade docs anyway.
Upgradig from 1.1 to 1.2 should give no problems. from 1.2 to 2.02 i had to remove the hosts contactgroups or somhing like that not much work really.
Just do it the "easy way"... make a second installation on the same machine on another Virtualhost.
When it's up and you are sure all is well just swap the virtualhosts names
Be sure to make a copy of the original nagios binary file... during the mae process one of the comands copies it's own nagios file to /etc/init.d/nagios and you need that one an the new one with different names to start stop the different instances.