MacOS – Roll back to Mountain Lion but don’t lose saved documents

The Time Machine backup should do a pretty good job keeping all of your documents and most of your settings and should be backwards-compatible*, but I recommend making a complete, bootable backup of your previous system so that you're guaranteed to be able to restore your machine to exactly as it was before updating.

I'm a fan of Carbon Copy Cloner (free trial) for making the backup. Back up to a clean disk or partition using CCC and tell it to copy everything. It will also help you clone your Lion Recovery HD onto the backup disk.

Once the backup is done be sure to try booting from it to make sure everything worked.

*Should be, but I haven't personally tested.

The method you mention first is your best hope for going backwards. It will not work well and many programs may have changed - if so, you will get an error message if the older program were programmed to detect this backwards migration of data.

The system stores data in databases scattered in some non-obvious locations:

• Contacts (Address Book)
• Calendar (iCal)

So, you can also export the data from these programs or check that the data is in iCloud or some other sync service. Then you can see if those programs work back on 10.6 - iCloud won't since it's 10.7.2 or later and Mobile Me is no longer working so for that sort of data, you need to export everything you need to move back to 10.6 (Bookmarks, People, Events) to your desktop and make one last backup using Time Machine.

Now, you mentioned iPhoto - that too could be problematic, but the good news is the OS update doesn't include iPhoto. As long as you run the same version of the app from before the downgrade (from 10.8) to after the downgrade (to 10.6) - your iPhoto library file should work just fine.

Apple's Migration Assistant won't move the program and it's support files and settings backwards from 10.8 to 10.6, so you will need the original iPhoto install and have to reinstall all third party apps that are more than a simple program you can copy from /Applications.

So to make this long explanation short:

• Apple's migration assistant will not help you move files or programs
• Time Machine stores backups in a plain folder so you can use Finder to connect any TM backup drive to a cleanly installed Mac and copy any and all files by hand - you just have to know which files to move and how to work through any errors when a file or database is too new for an older program to use.