@NSEU, MMouse, and vapilot2004.
As I remember the -200's that JANET flew did not have automatic switching. My NG737 school stuff from the mid 2000's all shows manual OXY/Boom switching as well, but a lot of the school was geared around BBJ1's.
So I would presume that it was either an option, or introduced after I went to the avionics school.