Ubuntu – Can you run unity8 as your desktop

No not yet. I'm sure we'll see some mockups, if not runnable code, in the coming month or so though.

In short, probably not.

Forgetting the desktop, there were a handful of applications that I had installed on a Lucid -> Maverick box that were removed by Natty B1 installation because of missing dependencies. These weren't mainstream applications - most of which are maintained and thus version compatible with the Natty libraries, and none of them were critical to what I do so I don't remember which were removed.

Unlike that other operating system, there isn't the overbearing market need to ensure that your copy of Lotus 1-2-3 from 10 years ago still runs on the new shiny.

If you've got hundreds of users, surely one of them won't miss their computer if you "borrow" it as a sacrifice machine. Failing that, carve off some disk, or get another disk (they are stunningly cheap these days) clone off your config and upgrade the copy to see how insufferable the damage is. It may also be useful in your environment to be able to dual boot Maverick/Natty as needed.

Added: You are asking for a guarantee that only empirical testing can afford you. Ubuntu is such a hypercomplex maze of interdependencies that given a huge static survey of your current installation, I'd be hard pressed to give you a definitive answer to what will change or break. If this wasn't so, you could just upgrade the packages you wanted. But you can't do that without entering dependency hell which the modern distros work so hard at avoiding.