I have seen a fair number of HDs which have been rechambered to accept .357, and in fact own one of them myself. It was something simply done, and many owners did it, or had it done by the local gunsmith. From a shooter perspective, there is nothing particularly bad about it, but it does negatively affect value if done on a high-condition HD. So be wary before you buy it. As previously stated, there are definitely several reasons to be suspicious about its originality.