^^^ - 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.