Hi Stevo,
The old BEAD 4 string is more common than you think. There was another thread on here which covered the same issue. I'm sure if you put tuning in the search at the top of the screen you'll find it.
I had the same thing, one of the bands I'm in tune to D because the singer can't reach the high notes. Then I found some songs had to go down to C so bought a RBX765 but couldn't get on with it so sold it and am in the process of getting another 4.
You shouldn't need a new nut to put bigger strings on, just file the grooves out a little. If you were to revert back to EADG you would probably need a new nut.