Thanks the for the spellchecker tips. I wish it didn't need the uninstall step, I always seem to lose something when I have to uninstall it :(
You might be able to get the spell checker to work without uninstalling. You could try using the installer, but you will need to grab a copy of a file called composer.xpt and place it in the "components" directory of your program installation. The composer.xpt file can be found in either the spell checker extension's site or in a firefox zip file. Again, I've heard that people have had extension problems with the installer version. Thus, you'd be better of trying to use the zip version. Just unzip 0.9 and run it before uninstalling 0.8. If this fails, bite the bullet and wipe the old version ;).
Is there something in particular that you seem to loose? Most of the information that you'd want to keep (bookmarks, etc) is located in your profile (for 0.8 it would be at C:\Documents and Settings\username\Application Data\Phoenix for NT/2000/XP or C:\windows\Application Data\Phoenix for win 9x/ME). 0.9 now stores profiles in the Mozilla/firefox directory under Application data. 0.9 will import your old profile to the new location on install, but I wasn't able to get the spell checker working without deleting my old profile prior to installation.
Personally, the only thing I save from old installs is my bookmarks file, since old themes and extensions sometimes break in new version of Firefox (hopefully this will change after 1.0). There is also a program called Mozbackup [backup.jasnapaka.com] that can be used to save your profile information.