Would the value of a 25-20 J Stevens A and T Co single shot rifle be reduced if it was modified to shoot 22 caliber cartridges instead?

It really depends on the overall condition of the gun. There are few guns of that type in original pristine condition. If it were in near new condition, it would definitely devalue it, to permenantly change it in any way. If it were in less than good condition then it really falls into a "fixer-upper" catagory, and the sale-ability or collectability may be affected depending on the quality of the work done on it. The gun can be modified to shoot .22 rimfire by replacing the barrel, falling block, and ejector/extractor without making permrnant modifications to the rest of the parts, so you could preserve the originality of the 25-25 caliber while shooting the affordable and readily available .22 rim fire. My advice is dont modify original parts, obtain a new made barrel and a rimfire falling block and ejector/extractor and enjoy it without ruining it.