either way, you have to take the dash apart and pull the radio out... weather you switch to a am/fm/cd unit, or add an in dash slave cd player. You can usually find the am/fm/cd units cheaper than the slave units, so that might be the better way to go.
The slave CD player mounts in the dash with the OEM cassette deck that you already have, all the functions of the cd player are on the cassette deck already, the slave unit just has an eject button on it.