All right, I'm going to weigh in on the "poa vs. regulation" discussion.
RP shotguns: Two barrel guns are (or should be) "regulated" (or whatever term you prefer) to shoot, more or less, to the same point at a given distance. For example, both barrels would print a pattern at more or less the same point of impact at, say, 40 yards. Different makers may use different distances, and/or may use different distances for differnt gauges. Different makers have different tolerances; I've heard that a gun was returned to Beretta with the barrels off a full foot off at 40 yards. Their reply: That's within factory specs. They can do it however they want to, and most can now assemble a gun that will shoot well without actually test firing it as the London gunmakers do. That's not to say they haven't figured it out, just that they are more thorough (and expensive) than most other makers.
As to point of aim - the barrels shoot where they shoot. "Point of Aim" (though shotguns aren't truly aimed, they are pointed) is then addressed though the stock (LOP, DAC, DAC, Cast and Pitch), and through the top rib (i.e. high at the back and low at the front will raise the POI in relation to the POA).
All these things have to work together to get the gun to "shoot where you look".
As to the original question - Briley (and other) install chokes into previously fixed choke guns all the time with no ill-effects.
