The original XD-40 was build directly on the XD-9 frame with no changes to the original frame. This is the reason the XD-40 has such bad capacity. Its not a true double stack since it has to use a 9mm width magazine bodies. But this made it easy to convert back to 9mm or 357 Sig.
The other reason you cannot convert most 45 ACP to 9mm is the breach face. The 45 ACP extractor would not touch the 9mm rim. The 9mm rim diamter is close enough to 40S&W rim diameter that the 40 S&W breach face and extractor could get a tenuous grab on the 9mm rim and for most guns extracts reliably. the XD-45 extractor would not get anywhere close to a 9mm rim and you would almost certainly have extraction problem.
You would also have a magazine problem as the XD-45 has a larger magazine well and magazine body then the XD-9/40 (or XDm-9/40) and thus here is no XD-45 compatible magazine that could feed 9mm rounds. The XD 10mm magazine might be an option but it might also need modified to reliable retain the 9mm rounds.
The nail in the coffin is, if you search the internet for XD-45 to 9mm conversion barrels you wont find one. So unless you know a good gunsmith that will make you a barrel your SOL.
22lr conversions almost always replace the entire slide and magazine to work and are alot more costly the a cheap drop in conversion 40S&W to 9mm barrel that many 40S&W handguns are capable of using. I believe their is a 22lr conversion kit of the original XD-45 (
https://www.advantagearms.com/mm5/merchant.mvc?Screen=CTGY&Category_Code=XDKits)
-rambling