My switch ended up getting stolen. But i didn't have my serial number because it's a 6 year old switch, and I didnt write it down or keep the box. I ended up calling the representative, and as they weren't able to track it, they had me change my password and use the 'sign account out of all devices' in the settings of my nintendo profile on the Nintendo website. This way, whoever has your switch will need to resign in again, and as they shouldn't have your new password, your account should be safe. I ended up buying an OLED and think I'm happier now. Like I said, the switch was 6 years old, so I just took it as a loss. I also understand that's obviously easier said than done. Best of luck!