the only difference between the 9 and .40 is that the 9 has a deep cut on the slide in the front,right where the large walther banner is and therefor less metal. there is a difference in wieght but it's minimal. all other dimensions are the same. there are pictures of both in the waltheramerica website but be careful some of the pictures are mixed.
as to the holster,it would depend on the holster. if it is specifically made with a molding for the cut for the slide on the 9,it may be a tight fit. some holsters do not go to those extremes. perhaps wetting the holster in that area and letting the molding readjust with the .40 in it may solve that but don't take my word on that as i am not to familiar with those specific holsters and i wouldn't want you to ruin them.
got to ride my motorcycle now,it's a great morning...peace