But so far all I knew was this fact, and I could give the relevant announcements in the Royal Gazette for this. In the 1943 announcement additionally some subdistrict and villages originally belonging to Ban Pong changed to the districts Photharam and Mueang Nakhon Pathom. The 1946 announcement also stripped off the subdistrict Khao Khlung and Krap Yai from Tha Maka and added them to Ban Pong, while the subdistricts reassigned to Photharam were returned in a second announcement in 1946. So the district not only changed ownership twice, it also changed its shape significantly - both northern and western appendix were added, while it lost territory in the northeast and southeast.
Now Khun Wisarut gave the rationale for this change in a posting at the 2bangkok forums:
Eer during the War time (1943-1946), Ban Pong District has been transferred to Kanchanaburi to facilitate the local contributions for the Death Railway Construction and Maintainance. Before the Death Railway, Ban Pong was [and still] a dropping point for Kanchanaburi - since there is a main street linking Ban pong with Kanchanaburi (now a part of Highway 323 - Saeng Xuto Road) and Thanon Songphon (Now a part of Highway 323 - built in 1915) linking Ban Pong with Nakhon Pathom.