Greg, I'd go with both of the above suggestions.
Big Dave's suggestion to keep version 2006 is an excellent one. I have always kept old Streets & Trips while installing a new version. If there is any problem with the new version, I still have my old version available. At a later time, when I'm confident the new one works fine, I can uninstall the old.
Ken in Regine suggests to back up your files saved in the older version. That is indeed a must. Just copy them all to a different folder, before you attempt to open any of your precious old files in the new Streets and Trips version. When you open a file saved in an older S&T, the new S&T will prompt you to convert the older file into a new file format. If something goes wrong with the conversion, you will still have your back up copies that will still be openable at least in the old S&T.
The above shouldn't really scare you, as in the vast majority of cases installation of Streets & Trips 2009 goes just fine, and opening old S&T files in new S&T version goes just fine, too. But it is always prudent to take the proper precautions.