On machine 1, I'm running Windows 10 Version 1909, build 18363.1139, with WSL1 and Ubuntu 18.04.

On machine 2, I'm running Windows 10 Version 2004, build 19041.572, with WSL2 and no distros yet.

I tried to migrate my Ubuntu distro to machine 2 by running wsl --export on machine 1, moving the created TAR file to machine 2 via USB drive, and running wsl --import on machine 2, but all I get is the message Unspecified error. So obviously I missed a step there. I'm afraid you're gonna tell me I have to upgrade Windows to 2004 on machine 1 and upgrade to WSL2 before I export Ubuntu. I posted this on the off chance that there's a way to avoid that.

EDIT: Full commands and results:

From machine 1:

wsl --export Ubuntu-18.04 Ubuntu-18.04.tar (using Command Prompt)
gzip Ubuntu-18.04.tar (using Git Bash)

After moving Ubuntu-18.04.tar.gz to my home folder on machine 2, I created an empty folder, Ubuntu-18.04, and entered these commands:

gunzip Ubuntu-18.04.tar.gz (using Git Bash)
wsl --import Ubuntu-18.04 Ubuntu-18.04/ Ubuntu-18.04.tar

The last command resulted in this output:

Unspecified error