What command or series of commands could I execute from the CLI to recursively traverse a directory tree and reduce the bit-depth of all PNG files within that tree from 24bpp to 16bpp? Commands should preserve the alpha layer and should not increase the file size of the PNGs - in fact a decrease would be preferable.
I have an OSX based system at my disposal and am familiar with the find command so am really more keen to to locate a suitable PNG utility command.
AFAIK the only PNG format that supports the alpha layer is PNG-24; Reducing the PNG to another format may require specifying a transparent color in a CLUT, which will not give you the output you want.
From the feature list on PNG's website:
... which I read to mean that anything other than PNG-24 or PNG-48 does not support full alpha transparency.