Yes, practically speaking, all you need to do is change the bbl and the magazine. However, you may experience some reliability issues. Maybe 5% but it varies widely from gun to gun. IF you experience malfs, there are 3 other parts that you can change. It may take one, two or all three but it varied from gun to gun. Those parts are the extractor, ejector and spring-loaded bearing. Once you change all these parts, there is no reason that won't experience legendary Glock reliability.
Also, it is not THAT necessary that you get a conversion bbl. The stock 9mm bbl work fine and it is cheaper. The accuracy won't be great but it will function fine. It will still shoot acceptably at typical SD ranges. Yes, the 9mm bbl is slightly smaller but it does not "rattle around". But you still do not particularly NEED a conversion bbl to shoot 9mm and shoot them pretty well. I cannot say with any degree of honesty that doing this would damage a barrel or gun. The difference in the 2 barrels is only about .020" and doesn't move too awful much. There are a lot guns out there that have more slop in that area than this. We have 4 rental guns that we do this on a regular basis and there is no measurable wear or difference in accuracy now than when they were new.
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OK, here's a little more on running a .40 with a stock 9mm bbl in it.
I conducted a defensive handgun class today and decided to try my G22 with a G17 bbl. I didn't change anything else and run Wolf 9mm ammo all day. I didn't have a single malf all day. I think the biggest concern is about accuracy though. I haven't YET sat down at a bench and really tried to do the best I could with it but so far (see pic below) it seems to be more than adequate for most purposes.
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My 13 year old son is going to run my daily carry G23 with my wife's G19 barrel in it all day tomorrow for the 2nd in our series of DH classes. He'll put at least 500 rounds through it and I will continue to shoot my 22 with the 17 bbl in it. I think by the end of tomorrow, we should have pretty well established the fact that the "conversion" bbl is not all that necessary.
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We went out today and conducted the 2nd of a series of defensive handgun classes. The class requires a minimum of 500 rounds. My son ran my G23 with a 19 bbl in all day today and put a little over 500 rounds through it. I ran my 22 with a 17 bbl for the 2nd day in a row but probably only put 200 or so through it. Not a single malf of any description in either gun. We ran Wolf, Independence, Magtech, Federal and PMC ammo.
We just got through measuring and comparing to other barrels, both new and used. and can't find ANY measurable difference in any area or any wear anyplace that doesn't normally wear.
Accuracy seems to be as good as any Glock from off-hand and kneeling. I haven't tried the "non-conversion" guns from a bench or machine rest yet but I plan to. However, although the 9mm bbls shoot a tiny bit high, I was able to knock poppers down with monotonous regularity at 50 yards and hit an 8"x12" plate at the same distance EVERY shot.
So far, there is NO downside to running a stock 9mm bbl in a .40 slide/gun.
