yeh when I upgraded my front bar I wanted to stick my old front bar on my dads S1 R33... but after some research found out you needed to change the headlights,guards and bonnet.
I'm sure you could stick it on there without doing these things but it would stick outin places.
If your getting the series 2 front bar for cheap or free then I don't see anything wrong in sticking it on the series 1. it looks better, specially if you didn't have even the stock front bodykit to begin with.