Pietta would be a steel sight press fitted to a hole in the barrel and staked in (although those I've seen recently don;t show any stake marks).
Don;t think there is any trick to getting the old sight off if it left a stump large enough to grab onto...you aren't trying to save the sight stub, so grabbing onto it with vice grips and turning it like a screw should loosen it.
Personally, I'd proably solder the new one in place rahter than stake it. Low temp solder, use a pencil to color all arround the barrel at the sight hole (solder and flux won't stick to the graphite coating), clean out the hole, polish the sight base, tin the hole, tin the sight base, flux it, set the sight in place, clamp it, heat it. Don't use toom much heat or you'll ruin the color. Make anti-scale compounds, but driving a wooden dwol into the barrel will work for this...will char and smoke, but will protect from scale at these temps.
If you want to make the right staking tool, then will need to make a steel tube just barely bigger than the diameter of the sight, shape it, harden it, and slip it over the sight to stake it all the way around at once.
OR you might take the oppertunity to add a low dovetail sight...will let you take care of windage (and Pietta should have used one in the first place). Once sighted in, can sile/polish the sight base to match the octogon shape of the barrel.
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Low tech. My buddy epoxied his sight in place, then pin prick punched arround it, and it hasnt come off (been about 5 years).