A G27 to G26 caliber swap does not require a complete top end swap. The difference between rim diameters is 0.03", or 0.015" radially. Put into almost positive numbers, that's approximately 1/3d of a millimeter. Hopefully this gives you some idea just how small is the difference! Even if the 9mm rim moved laterally away from the extractor as it possibly could, it cannot escape being engaged. Additionally, because the ejector IS pushing the round away, this means it's pushing it directly toward the side the ejector is on.
I would recommend you save the money on a new slide and buy the dedicated 9mm "conversion" barrel unless it's outrageously priced, in which case I'd buy a plain old G26 barrel. Why? Again, the ONLY difference between a so-called conversion barrel and regular is the width of the barrel hood - that little tang that sticks out the back and indexes into the slot running up the breech face.
The hood is intended to create a mechanical alignment between slide and barrel for repeatable accuracy, so it does have a function, but this is more applicable to guns of the 1911 type which use rounded profile barrels and internal lugs.
On guns like Glock, the monster-size, "square" chamber profile (external) fits snug into the slide with ample lateral support to establish and maintain alignment. The gun will work perfectly fine with a standard G26 barrel which will have zero negative impact on the gun's use for SD or CCW.
The first, best thing to do is install the barrel, load up some 9mm and shoot the gun. THEN start worrying about what to do IF it doesn't work reliably.
If the gun cycles reliably you're done.
Modifying the extractor on a Glock is almost too easy. Remove all the components. Grab the extractor and locate the tiny pad on the inside that contacts the slide - this is what limits the extractor from rotating further in. Grab your dial calipers and mic the thickness of the extractor right at that tiny pad. Then, using a small file, give the pad a few strokes - remember you're only trying to allow the extractor to rotate in a TINY amount! Check fit using a 9mm cartridge rim inserted into the breech slot under the extractor and feel how firmly it grips it.
Give it a few more strokes with the file, check with the micrometer and once you've thinned it by about 0.001" you're done for sure.
To give you some perspective, I've been running a .40S&W slide on my CZ-75 fitted with .22TCM9R barrel. The rim on a .22TCM9R is about 0.020" smaller than the 9mm rim, so there is noticeable "play" between the rim diameter and breech face. The extractor has been modified as stated above, and the gun has its stock .40S&W ejector. It works superbly.
