FWIW, the backup function in my old zaurus SL-5500 backed up everything I had installed. It restored everything withouth problems after flash OS updates, this was extremely convenient.

Now, there could be compatibility issues, but IMO it's better to restore, and then remove non-working apps afterwards, instead of having to go and re-install everything by hand. (And here we're back to the old OS2006-2007 discussion, but really, I've got apps. back from 1992 that still works on my PC linux, although some don't. But everything from, say, 1994 works. Getting old apps to work with new OS'es is _not_ a big deal. The other way is a different story. But of course, coming from smartphones doesn't teach this..)

On my own N800 I'm going to set up an extended backup system of some kind, before the next OS version becomes available.