After being burned a few times on upgrading a few different themes, I took this to task to figure out a better way to get this done without losing access to the site, and being able to fall back easily. I have had this happen to me with other themes also, not just Suffusion.
The first few times I got my previous folder back by using the Time Machine that comes with Mac OS X, since I was testing on the local machine. I realized quickly that this wasn't going to be the solution for my installations on a hosted environment. So here is the process that I ended up using, and should work for any platform, both local and remote. I'm hoping this helps someone.
When an upgrade is made available, I will first make a copy of the theme. The name of the copy doesn't really matter. You can locate it in the Manage Themes easily by looking at the section labeled "All of this themes files are located in". You will see the name of the copy here. This is an instant backup if things go bad.
The second step will be to activate another theme. Most everyone probably has the default theme still installed. By doing this, you can now upgrade the theme in question without taking the chance of losing access to your installation through the Admin section of Wordpress.
Now in the Admin Panel you can upgrade the theme without having it being the active theme. The trick here is to test the upgrade before activating it. How? Click on the Preview option that is available, after the upgrade is done. If you see something that resembles your site, then the download was probably successful. If you get a blank page, this indicates the the WP servers gave you a corrupted copy. I did notice that clicking on the Home button in the menu took me to my Blog page, and not my regular home page. I use WP as a CMS, and not just a blog tool. The other menu selections work fine.
Did you get a blank preview page? Now is the time to initiate the simple recovery. Delete the theme folder that is now corrupted. Make a copy of the copy and rename that one to the original name of the theme. You are instantly back in business with the previous version, and you still have your backup.
Remember that you are doing these steps while you are using the default theme, not the one that you are trying to upgrade.
Next step - try the upgrade again. Test it again by clicking on the Preview and see if you get a good preview screen. Eventually you will get a good download from a reliable WP server and you can now activate it and move on.
And here's hoping that WP can find out why this is happening.
And thanks for a great theme and all the work you have put into this!