Mike,
Based on my past experiences with MS Office products, I would say you can indeed install the 2000 Professional seamlessly over the other version you have.
You would be well served to snoop around on Microsofts' web site, and get the definitive answer. It's always a good idea to find out all you can 'before' you commit to a particular upgrade. Then there are no surprises.
However.....
Since I prefer to keep my computers very lean and mean, and as fast as it can be, I would do it like this:
Make an image of the drive. Use Ghost, or Drive Image. If anything should go wrong, this allows you to install your backed up image and go back to where the system was before the upgrade.
Since the 2000 Pro is a full version, back up all your document files, ( .doc, .xls, .mdb, etc) to a separate folder on your hard drive. (This is an additional data backup for the Office files).
Completely uninstall the Small Business version.
Run a Microsoft utility called Regclean. It is available on their website, or I could email it to you. As the name implies, Regclean inspects the system registry for unlinked references, (what I call deleted program remnants). You need a lean & mean registry for optimum system performance, and this tool is what does it.
Now defrag the hard drive. Defragging ensures that when the new software is installed, it will be installed contiguously.
Then, you would freshly install the 2000 Pro, and finally, restore your backed up data files.
After verifying that everything is working as expected, make another image of the drive. Use Ghost, or Drive Image, as I mentioned before. Put in on a CD, if you can, or at least on a different partition of your hard drive.
The bonus of this all is what I will always preach..."BACKUPS!!!"
I have condensed the steps above to a high level description, but it gets the point across, I think.
Doing it this way helps keep your file system in optimum condition. (optimum = fast) Following these steps for installation of all applications is even better.
I know this deviated a little from the original question, hopefully it helps somebody.