Yep, in S&W metal-framed pistols and the M&P pistol series are tapered and the sights go out left-to-right and back in right-to-left.
Not a steel punch, not for hammer blows ... a brass drift just small enough to fit in (and through) the dovetail cut. The pressure is focused against the side of the sight base, not the post itself (which will probably leave brass on the slide, if you drag or tip the drift, but which should wipe off with some regular solvent), because the power behind hammer blows can sometimes snap off a post.
While I used to use a 4 oz ball peen hammer and brass drift for sight work, I eventually started using a sight pusher when it came to night sights so I didn't damage the glass capsules. I was lucky that I never broke any night sights when I used to install them using a hammer & drift (and even had fit some sight bases themselves in older models), but I eventually decided to stop risking breakage of the night sights, which are expensive.
For installation I usually use a drop or two of oil in the dovetail cut and tap the sight base with a plastic mallet to start the sight base into the cut, making sure it enters evenly and straight. Then I switch to a drift and ball peen hammer or a sight pusher. Again, the drift (because of hammer blows) should be against the base of the sight and not on the post.
I had to move my front & rear sights in one of my M&P pistols, but I was able to do so using a sight pusher without undue effort.
If you've never had experience in properly removing & installing a sight in a slide's dovetail cut, you might consider asking for some help from a local smith. The cost of replacing a sight damaged by improper removal or installation, or risking potential damage to a slide (for whatever reason or circumstance that might occur) would seem to make some professional assistance prudent and cost-effective.
Just my thoughts.
