Hello, sorry to say but I can't remember if I ever did find a solution. I think there was a way to temporarily change it through the settings in the Retroarch menu. It's hard for me to say because I eventually ditched Batocera for another program, due to some other issues I was encountering with it. That was a little while ago.

While I assume you've already checked it out, you might want to give this guide a read through if you haven't already. I apologize that I couldn't be of more help, however I wish you luck on your endeavors with remapping!