We already know this about previous versions being on the computer.
What if someone had to reformat and doesn't have to older install files? How about actually answering the question for once and for all?!!!
Here is S3D-Jason last response.
"When you install a new version of Simplify3D, the old version will still remain on your computer. For example, if you were using Version 4.0.1 previously and you now install Version 4.1.1, you would be able to find both versions at the directories below, since the prior version is not modified by installing the new one.
So if you want to run the old version, you can just navigate to the correct directory and double-click on Simplify3D.exe in that folder. "