To convert to/from indexed color, use the Image > Mode menu, IIRC; there should be options for indexed color, RGB, and a whole bunch of other stuff. Most likely the file's in RGB, though I'm not too familiar with the TGA format so I'm not sure. As for swapping the palette, I don't have Photoshop handy but there's probably an "edit palette" or some such command on the Image menu somewhere... is that at least a bit closer to what might help you?
