As other have said 9 times out of ten you won't have any issues just using a conversion barrel, and 9mm magazines. However, you should change the EJECTOR (not the extractor) to ensure that the casing ejects properly given the 9mms smaller diameter.
I have a KKM conversion that I use in my G27, and I haven't had any issues other than irrational ejection using the stock .40 ejector, but its not something that I would be my life on.
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