If you open notepad and then open the *.sai file with notepad, you will be able to see what version the file is operating under. Then open that version of the program and open the file. If you open the proper program version for your file, then open the file, there is no way to have the file convert to a different version by accident.

Personally I never, ever open a file by double-clicking the *.sai file. It tells Windows that Windows knows more about what you do with your's and your client's data files than you know.

Windows holds a file association for the *.sai name and that it is assigned to a specific program. In the case of Sage 50, it is my experience that the file association is assigned on the installation of the program. So the last program installed will have that association (association means if you double-click the file, it is automatically associated with a specific program and Windows will find that program and start it to open the file). The file apparently is associated with the 2013 version of Sage 50, probably because you installed that one last.

I opened the file in Notepad. It says 2016.

So I need to buy the 2016 version in order to open it?

If the file indicates that it's in the 2016 version, then yes, you would need to purchase Sage 50 2016 to be able to open the file. One other bit of advice, double-check the .SAI file in Notepad to make sure that you purchase the correct program version, that is to say First Step or Pro or Premium or Quantum.