Hi Damien
Im often proven wrong in my thinking so take this advice with a grain of salt. First rebuild you lib with all three steps, see FAQ, then update your AP Vault. Have a good back up of your entire system. Up date the os from snow leopard skipping lion if possible and run Ap to confirm all is working. after all that upgrade Aperture and don’t be surprised if the library will need rebuilding after the AP upgrade. Good Luck and have fun…after all you have backups. Cheers “Columbo Style” advise

Just offering this for thought … You may want to consider starting from scratch making the leap from Snow Leopard to Lion/Mountain Lion (most likely to Mountain Lion unless you have the old Lion installer on hand since it is not available from the App Store)

Much has changed in Lion/Mountain Lion compared to Snow Leopard. I recently went this route and decided to leave all that legacy stuff behind. There is a tendency to accumulate a bunch of unnecessary supporting files for apps and utilities you may no longer be using. Often after we download and trial an app or utility, the “uninstall” or simply dragging the app to the trash can leave behind a myriad of supporting files, caches and other files we no longer need.

If you install in place with a new system, or update using Migration assistant, it keeps everything … even those files you can no longer use for legacy apps that can’t run on the new OS versions … By formatting my startup drive, performing a new install of Mountain Lion, then manually installing all my apps … then manually drag over only those Application Support files, preference files and documents from my cloned backup … I ended up saving considerable amount of space and eliminated all the extra clutter …

Before you start, to make a good clone of your drive as a safety net … be sure to keep this clone on hand until you are sure that you have everything transferred that you need. Of course laying the groundwork to make sure I had all the non Apple app installers on hand and the relevant serial numbers on hand for the installs takes a bit of time. Also … make sure you deactivate any Adobe apps (or other apps that may require deactivation before you start the process) BEFORE you format the disk …

Now my systems are lean and mean and only have the data needed to get the job done without all the extra erroneous crap I no longer used …