Yes, I have save a PPT file to PPS before, but vice versa, I don't have the experience. As a curious man, I tried for a while and finally find out that: if your PPT is make in PowerPoint 2003, you can directly change the suffix.But for PowerPoint made in 2007 with PPSX or PPTX format,it doesn't work. Now I want to share with all of you. It is quite simple actually.
The simplest Method 1:
1. Run PowerPoint, click "Open"

2. Find the correct path of the PPS file
3. Click "OK", it will be opened just like PPT
4. "Save as" to a PPT format.
Another Method which I find first:
1. Run PowerPoint
2. Drag the PPS file into PowerPoint, you will see the PPS file will like a picture
3. Right click on it, choose "Presentation Object" --> then choose "Open"

Now, a new PowerPoint window will pop up, and that is a PPT file, you don't have to "Save as", it is a PPT file now.
Easy, isn't it? There are thousands of small tips and skills when using PowerPoint, as long as we learn more, we will find PowerPoint easy and interesting.