Personal Historian 2 installs as a separate program and does *not* overwrite your version 1 program. After installing version 2 you will convert your version 1 file and will then have a completely separate file in your version 2.
At this point you can either keep version 1 on the computer (both version 1 and version 2 can be run at the same time with no problems), or you can uninstall version 1 from your computer if you don't need to use it anymore.
Note: Personal Historian 1, 2 and 3 each install as separate programs. No file conversion is needed between version 2 and 3. File conversion is also needed between version 1 and version 3.