I think the main way the game revolutionised the series was by introducing it to a wider audience. To my knowledge, none of the previous Final Fantasy games actually got a European release (Wikipedia certainly doesn't give any release dates for European versions of FFI, FFIV and FFVI). In a way, I'd say the game revolutionised how people viewed RPGs and the audience itself to as large (if not larger) extent than the series itself. Though FFVII has, obviously had some noticeable effects upon the Final Fantasy series itself.
