So Apple came out with this shiny new operating system with a really cool name...LION. And you are thinking to yourself, "Hey, I'd like to install that new OS on my computer." OK, I can dig that. But there are a few things you should do FIRST, before you install. Especially if you use this computer to edit with Final Cut Pro...and depend on that machine to earn your keep. Because if you install LION, and things don't continue to work as well as they used to, then that will cost you in lost time that can lead to lost money.
Step #1 - RESEARCH!!!
You need to look into whether or not your current applications will even work under the new OS. You might be shocked to find that many of them won't. For example, Final Cut Studio 2 will work...but the INSTALLER will not. Because the INSTALLER isn't Intel native...it requires Rosetta to work, and LION doesn't have Rosetta. For this reason, MANY applications that rely on it won't work. Adobe CS2 will not work on Lion. So do your research to find out if the applications you rely on will work on the LION OS. Make sure that the hardware you rely on for video input and output (capture cards) have drivers for LION.
Step #2 - CLONE YOUR WORKING SYSTEM!
Clone your current working OS drive. Get a cheapish firewire drive...something that the system can boot from...and use Carbon Copy Cloner (bombich.com/download.html) to completely copy the current working system drive to another drive. This way you have a copy of your working setup in case LION doesn't work out. If things don't work, just boot from that drive and erase your main drive and clone it back. You will lose a day, tops. And this keeps you from needing to reformat the drive, install the OS fresh, and all the applications fresh, then bring back all of your files, set up all the applications properly again...stuff that can take days.
You can back up any files you want manually as well..but the clone will have everything in case you forgot something.
Step #3 - DO A CLEAN INSTALL
Boot from the LION drive and then ERASE your system. Wipe it clean. Then install the Lion OS fresh. This ensures that you are getting the best possible OS install. Installing on top of existing OS might work...it does for some...or it might not. Some people report issues, others do not. But doing it completely fresh ensures that you have the best possible install. After the install, check for any possible updates with the SOFTWARE UPDATES in the System Preferences.
Step #4 - INSTALL ALL OF YOUR APPLICATIONS FROM THEIR INSTALLERS
DO NOT use Migration Assistant for applications. Install them fresh from their install disks or installer files. Because many of them, like FCP, install bits and pieces of the application throughout the OS, and Migration Assistant might miss those files. Correction, WILL miss those files. So if you want to have a good working application, install from the installers. You can migrate your files if you want...or manually drag them from the clone drive...but not the applications.
Then run the updates for the applications (if they are Apple apps, use the SOFTWARE UPDATE in the System Preferences) to update them fully. And install any drivers, firmware, other bits for other things on your system. Like Capture card software (make sure you get the latest versions of the drivers) and plugins and graphics cards drivers.
If I forgot anything, please feel free to comment and add that to the list.
Personally, IMHO, if I have a good working system, I do not update. Because my system is working, and I might not need any of the new things the new OS offers. I recently upgraded to Snow Leopard only in March, because one application I relied on...the new version of it...only ran on Snow Leopard.