Is it possible?

Anyone have both a .45 and 9mm to compare them? Does the 9mm slide fit on the .45 frame?

I think they are the exact same except the hole in the 9mm barrel is smaller of course but the outside diameter is the same as the 45.  Now there still may be other differences such as the extractor and ejector.

^^^ - this. Although assuming you drop an entire 9mm slide assembly onto a .45 frame, the extractor shouldn't be an issue, but the ejector might be a problem. Without having an XD-S .45 to compare to my XD-S 9mm, I can't say for sure.

However, there's a different issue:

http://xdman.com/blog/2013/6/1/existing-pearce-grip-for-the-xds45acp-also-fits-the-xds9

Notice the difference in the front-to-back length between the .45's mag vs. the 9mm's mag. I just looked on my 9mm -- there are ribs that protrude from the back of the magwell wall that I'm guessing are not on the .45. When a mag is inserted in an XD-S 9mm, there's a noticeable gap between the back of the mag and the rear magwell wall...the size of the gap is suspiciously close to the difference in the front-to-back lengths between the two calibers' mags.

So my edumicated guess/SWAG is that a simple conversion isn't possible.

There's been a decent amount of talk about this on xdtalk.com in the XDS section and the general consensus is that they're too different for a simple slide swap conversion.


edit:  Mags from .45 will not fit 9mm frame and vice versa.  A slide-swap conversion would not work.

No, No!  
Convert it FROM a 9mm to a proper .45

Don't downgrade it!  Upgrade it!

I can confirm that the mag wells are indeed different, and swapping mags isn't possible.