as i'm upgrading the web server, one thing i've learned that i want to pass on, that i'm reminded of each time i do something like this:
the safest path to doing major changes and experiments, whether to a website, or an operating system, etc.,
is to make a mirror first of your current setup, and do the first upgrade on the mirror.
in the case of the website this means setting up a copy of the website on an isolated space, re configuring it as current setup, and doing experiments there. i am about to experiment on some php and mysql settings now and this is a safe way to do that too.
in the case of computer, i use a hard drive rack (see our backup review for some tips), and true image to let me clone the hard drive, and then work on the clone until i know everything is ok (you could just use an external drive to image the main drive and roll it back if you have troubles, but the racks and duplicate hard drives let me experiment without having to roll back).
there is also another MAJOR advantage of the idea of creating clones and experimenting on the close, which is that you really learn how to go through the steps of recreating your system, which can be super useful.
this is the same benefit you get from installing a new clean operating system every once and a while, it gives you a chance to go through the steps and avoid the mistakes and make sure you really do remember how to configure everything the way you like it. its better than waiting until you have a catastrophe and then trying to remember how on earth you ever configured everything.