Not sure why it has to be. Being a superpower, it already defies logic. Why is it necessarily painful? I don’t see why the brain can’t temporarily shut off pain receptors if it’s already doing something fantastical.

I’m reminded of the Animorphs books, where they describe the process as grotesque and the odd sensations they feel.

The Animorphs series explores this quite well. It’s not actually painful for them, but should be and is plenty disturbing.

Only the first time. Then just leave out the nervous system going forward?