I have removed and replaced a rear sight on a CW9.
I 1st drifted it for windage, then eventually removed it and turned it around because that cut out in the rear was distracting.
The rear sight drifted out from left to right as I recall and I would measure the dovetail with a caliper to determine which is the widest side.
I have also installed XS big dot sights on a Glock, with that rear sight a slip fit in the dovetail with set screw retainment.
Now, I have never delt with the PM9 FRONT sight dovetail design and that may be the difficult part.
I believe the XS sight is a good choice given the usual purpose of the PM9, with the smaller Dot being somewhat better for pocket or holster carry.
XS may be able to answer any install questions.
And if Kahr does it, you need only send the slide, which is not considered a gun for shipping purposes.
YMMV
Smiths shy away from type work, especially if you supply the sights.
It's a $10-$20 job and if they break a vial during the install you will want a new sight or them to pay YOU for the broken sight.
Often at gunshows there are people who sell and install (usually free) these type sights, while you watch.
Watch them do someone elses to decide if you want them working on YOUR gun...or not.