You will need to change the ejector on the frame to convert to 10mm from .45ACP. Barrels are available for the Para in 10mm from various sources. Just remember you will need a Para/Clark style ramped barrel. Brownells lists a ramped barrel in 10mm from Clark for $170.50. Part #181-200-106. You can also order one from BarSto precision for $185. http://www.brownells.com/ http://www.barsto.com/
You'll need to use a 40 ejector, but you'll need to remove about 12 thousandths of material from the right side to allow the 45 slide to move fully to the rear without hitting the nose of the ejector. Just take the 40 ejector and lie it flat on emery cloth and scrub back & forth. Measure the front & back with calipers once in a while & if they are different, turn the ejector back to front on the emery cloth & scrub some more. After about 10 thou. is removed, set the ejector in the frame & run the slide back. If the slide stops because it's hitting the front of the ejector,remove a bit more material until the slide passes over the ejector without making contact. Because the 40 is a narrower round than the 45 the ejector is moved that little bit toward the center of the cartridge. If switching from a 45 ACP frame to a 40 S&W conversion, you'll need to get a 40 S&W ejector. If switching from 40 to 45, just do the bit described above.