Thanks so much everyone for all of the great feedback. I knew about needing to use steel shot for duck, but hadn't even considered how this might affect the barrel of the older gun (lots to learn, still...).
As a couple of you guessed, based on sentimental attachment alone (my gramps was the man), I won't be modifying or reselling the Model 12. I know it was one of his favourites. I'll need to bite the bullet and buck up for the more expensive loads if I end up using it for duck hunting.
Your input raises a couple more questions for me. Again, excuse these questions if their answers are obvious to you guys:
-Would the softer non-tox loads pattern much differently than similar-type lead shot? In other words, would it be feasible to practice/target shoot with a matching lead shot and save the pricey stuff for the pond?
-More generally, ammo aside, thoughts on the 12ga Model 12 Featherweight with full choke for a beginner duck gun? I understand that so much depends on distance and what type of shots I'll be taking, but as a versatile beginner's gun, is it generally considered to be more favourable to have a Modified or IC choke?
Thanks again!