If you right-click the icon, and choose properties, you should be able to set the shortcut to whatever you want. I use Firefox as my browser. I have an icon for it on my desktop. To change it, I would right-click it and click Properties. In the Application tab, there is a an entry called command. Put the full path to the command there... "/usr/local/seamonkey/seamonkey". Click OK, and you should be good to go. I have KDE as my desktop, so if you are using GNOME, it might be a little different, but similar enough.
As for upgrading to the newest version of Fedora, you would prob'ly have to do a re-install... Going from 4 to 8 would be too much of a jump, in my opinion, to do it with an upgrade. If you have a separate /home directory, you wouldn't have to format that, so your settings and files should be OK. Of course, you should always make backups of anything you can't afford to lose.