Changing aquarium filter?

At the moment my tank has a internal power filter but the wiring was a bit dodgy so it needed changing, anyways i bought a tetra in plus 400 power filter to replace it, but will i screw the tank up, seeing as the beneficial bacteria from the filter media wont be there anymore, i was going to cut the media up and stick it in the new one as its established but the fish store guy said thats its not compatible and that i shouldn't do it, if i had more time i would run the new one with the old, but i haven't got that luxury, thanks for the help