Helpmate in 2
b) Rooks become bishops
c) Rooks become knights
d) Rooks become pawns

a) 1.Rb1 Re3 2.R4b2 Ra4;
b) 1.Ka3 Bg5 2.Ba4 Bc1;
c) 1.Sc2 Sd5 2.Sca1 Sc3;
d) 1.Ka3 e8R 2.Ka4 Ra8.

It was quite an achievement to find an arrangement of the pieces which provides unique solutions for all four parts.

Richard Stein: I fancied this problem, it provided some merry entertainment for an evening. I actually solved the parts in reverse order, as I found the harder parts to be earlier and the easier ones later. The effect of the pawn promoting to the rook of the original diagram should not be lost on the solver.