Not necessary. A single neuron has a very limited range of responses, so a meatbrain must be physically reconfigured to substantially change its function. A properly-designed computer can emulate a huge range of different systems in software; adapting outputs in response to changing data is a fundamental characteristic of software.
No True Scotsman. Anything Turing-complete is a computer, any computer can be programmed to solve any mathematically-expressible problem if there is sufficient time and storage available, and any problem involving the motion of matter and charge can be expressed mathematically. There can be no doubt that it is physically possible for a sufficiently powerful computer to fully emulate a human brain. Whether we’ll ever manage to build that computer, whether we will ever understand the brain well enough to write the emulator, and whether it can run in realtime is open for debate. But if a time traveler handed you the program today, you could run it on any sufficiently powerful cluster of modern computers.
