converting an unconditional into a conditional branch

Hi,

I'm learning to use the llvm API, but I got a question for which I haven't
found a good answer yet:

What is the best way to turn an unconditional branch into a conditional
branch? All I want to do is add two more operands, a successor and a
condition. But no matter how I try to do this any one of
setCondition()/setOperand()/setSuccessor() causes an assertion like these:

Cannot set condition of unconditional branch!
Successor # out of range for Branch!
setOperand() out of range!

Can this only be done by completely replacing the unconditional branch
instruction with a new, conditional one or is there a method with which I can
implicitly turn an unconditional branch into a conditional one by adding a
condition and a successor?

I'd be glad if anyone could give me a hint on how to do this most elegantly.

Regards,
Florian

Replacing the unconditional branch instruction is appropriate here.