We could use some more information.
However, I can only assume you are concerned about this upgrade because you have an application on there that you think could break in some unexpected way. In which case I have to wonder why you have not set up a vm and tried this application there as a test.
Next, I also have to wonder how you are backing up your server in the first place, and why you don't already have a DR solution thought through.
Barring that, you can do as others have suggested and clone the machine, using Ghost or similar depending on what Operating System you are using.
Really though, there is zero penalty for failure in a test scenario, I don't understand why you haven't started there.
There are other ways out of this jam, but they all require you tell us more.