Hey first off how old is your 12 disc changer, I sell some car audio and find that the older changers are not compatiable with the newer decks so you might want to check that out first!!!
Well the difference between the 2 are that the 7400 has only front and rear preouts, but the rear is non fading so you can use it as a subout easily. The 8400 just has a funkier display and is all motorized and stuff. Plus it has a front, rear and sub preout. So your only choice is to find out what you really need your deck to do. The difference in price here in Canada for the 2 decks are going to be 400, with that extra dough you could buy your self a decent sub and amp too.
just my opion
