Let's say you undergo a process to slowly use your brain's plasticity to get closer and closer to perceiving as a bat. Well, does that help me? Am I able to imagine what it is like to be a bat? Of course not. Well, what if you could talk to me and describe it, then would I have a true understanding of the experience of being a bat? Still no!
Nagel's point isn't about whether we could do anything to make ourselves into bats, it's rather that, when we don't have that qualitative experience, we cannot truly understand what it is like to have that experience.
If anything, the argument works even better now that you've gone and given yourself sonar: you can talk to me, unlike that dumb bat, so you can describe your experiences to me, and yet I will still have no true understanding of the experience itself.
That is (a part of) the Hard Problem of consciousness: that my studying and examining and learning about your conscious experience still does not allow me to understand it.