Well I figured out a hacky way to do this. If anyone knows the "right" way to do it, let me know.

Here is what I did. The location of the texture is specified relative to the location of the .CAS file, so if you just put the .CAS file in a different folder, and the new texture in a folder called "textures" within that folder, then everything works. I think this should work for other objects as well, like chariots etc.