I'm running a Mid-2011 Macbook Air, my first Mac. When I bought it, it had Lion pre-installed. Yesterday I picked up an external hard drive, and made my first Time Machine backup. In the next few days I plan on going to Mountain Lion, but am trying to be cautious.
If I have issues with Mountain Lion, and want to revert to Lion, what steps can I take now to ensure that I don't lose anything when going back to Lion? Will a Time Machine backup + the Lion Recovery disk maintain all my documents/data/settings? So, I'd have a Time Machine backup of my post-update Mountain Lion system, will Lion/Time Machine be able to take all the documents and data that were created under Mountain Lion and restore them to my system after reverting to Lion?
Essentially, I'd like to know what other steps I can take now, while I have Lion, that will make this hypothetical reversion a smoother experience. Is my plan of Time Machine + Lion Recovery disk fundamentally flawed in some fashion?
edit: As discussed in the comments, I've made a TM backup, and made a Lion Recovery disk. I went forward with Mountain Lion's install. I see in the Mac App Store, that I have ML available to redownload, but there's no option there for Lion (which I was sort of expecting, but now have confirmation). Looks like the only way to reinstall Lion at this point is to use that recovery disk.
Best Answer
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.