If I install it this way and restore my backup from v8.28 will all of my setting remain?When upgrading from 2.8 to 2.9, do NOT use the all in one package. The first time it boots as the version upgrade occurs, it ONLY installs the system package from the all in one package, nothing else. If you can get access to the unit again, install the all in one package a second time and it will then install everything, but the only access you've got to the box until then is via the ethernet ports. One other thing, you'll also discover that all of your wireless config has vanished and that you need to redo it.
The way around this is to install the individual packages during the version change. They all get installed then correctly and the config stays intact.
I spent a few hours recovering a remote AP yesterday and then experimenting in our office to figure out what was going on and what would work correctly.