Is it possible to unload built-in kernel module? I have a kernel with built-in module that I want to replace. I have built a .ko and tried to insmod it, but it failed since the kernel was already built with it.

Is it possible to do this?
No. Built-in drivers aren't modules, they are built-in (hence the name). If it's not showing in lsmod, you can't get rid of it short of building a new kernel.