You can't change that into a good turkey gun. You CAN kill turkeys with it. I've killed probably a dozen turkeys with 2 3/4" shells, and guided a few more. No question it's possible, no question you're giving up range to the 3 1/2" hevi-shot loads.
A good turkey gun will shoot at least 3" shells and preferably 3 1/2". It will have a short barrel and tight choke. It will also be camo or flat finished. The overall gun should be light weight, especially if you have serious ground to cover. If you're always going to drive to the same blind on 80 acres it doesn't matter much, but if you're covering thousands of acres every spring, you'll want a light gun. I consider a sling to be a requirement, but I cover lots of ground, a significant amount of it on my stomach. Sneaking in field for afternoon hunts and out of fields after dark or late morning often requires crawling.
My advice: I've got the 870 turkey gun and I don't particularly like it. I use it, and I've killed lots of toms with it, but it's a poorly constructed gun with bad balance and a wicked kick. It doesn't appear to leak gas like my old 870 did. I've also got a 24" Benelli SBE II and like it. It's my wife's primary gun, and I also use it as a loaner. I'm not an auto gun guy. I looked at trading the 870 for the BPS turkey but the BPS was more than a pound heavier. In a pinch I have used my 3" BPS but it too is heavy, and can't shoot those killer loads. I used to use a BPS 10 ga. but it was way too heavy. If money is no object the SBE II is probably the best choice. If money is tight the 870 or nova would be my advice. If you're stuck with only the one gun, you'll need a full or super full choke and some of those 1 1/2 oz lead 6s or the hevi-shot equivalent.
