if while you slept somebody made changes to your brain at any smaller level, you wouldn't wake up thinking "I feel weird".

I have no reason to assume that you couldn't replace me entirely, piece by piece. After all, I have different cells now than I did previously, and will have different cells later, and all the while still perceive myself the same.

The only thing weird here, is the idea that I would somehow notice. I mean, if I could notice, it wouldn't be a very good replacement, would it?

(Actually, given my experience with mind hacking, my observation is that it's very difficult to notice certain background characteristics of one's thought processes, such that even if a machine translation did introduce a systematic distortion, it seems unlikely to me that anyone would notice it in themselves, at least easily or at first!)