Reloading Function from a bitcode file.

I've been trying to update a Function within a Module from a bitcode
file so that I can use recompileAndRelinkFunction to update the
function. It seems that after creating a new BitcodeReader regardless
of what I do, I can't seen to update the functions that exist within a
module, or even replace them if I manually remove them. Is there
something fundamental that prevents doing this, or am I missing
something more minor?