The point is, that by applying the Stereo Expander you are going from 24 bit to 32fp because that’s how WL works internally. That’s why dithering is recommended to go back to 24 bit and save the file.
Yes, it will change the bit depth but nothing is done to the file except adding zeroes, so basically the information is still contained in the 24 bits (it works a bit different with floating point, but let’s leave that aside). Also, if you change the properties from 24 to 16 bits, the lower 8 bits are simply cut off (trunctated). Not recommended to work that way.