They have a different advance curve. The 400 reaches full advance (35 degrees) at 6000rpm, the 600 (36 degrees) at 4000rpm. The difference in full advance is probably not an issue as it it probably determined by the flywheel rather than the CDI, but the difference in advance curve will make a difference. I have no experience, but I'd guess you might get away with the 600 curve on a 400 at the expense of premium petrol. Vice versa, it'll probably run but might overheat and you will probably lose some midrange performance.