I have done that with ProSeries before and moving the program to an externs drive does not work very well. It relies on some system files and registry entries that don't transfer over. It will open, but it glitches or freezes or crashes. 

The better way to do it is to back up your older data files on a portable drive, then uninstall the older ProSeries years you do not use. This maintains a lighter system, with your older work still being held safely.