You would be switching distributions so there would be no 'upgrade'. you would have to re-install the new OS from scratch.
If you have a second hard drive or un-partitioned space on your main drive you could setup a dual boot system. One that boots Freespire AND another Linux distribution, so you could try it out to see if you like it better or not.
Very few Linux users stick with the very first distribution they try, or at least they may try several others before coming back to what they started on. It's a very common thing.
I would actually suggest you try out several other Distros to see what is available. Fedora, Suse, Madriva, Ubuntu, are all very popular