I got it
- give your root user a password
- reboot
- when choosing Qubes OS boot in grub, type e to edit
- in the “module2” line, add
single
and press ctrl+x to boot - enter LUKS password as requested
- enter root password as requested
- type
usermod -l newlogin oldlogin
- type
exit
- done
Basically, you can also do it live but it’s dirty
- sudo -i
- vipw and replace your username with the new one
- vipw -s and replace username
- modify
/etc/group
,/etc/subgid
and/etc/gshadow
to replace the old username with the new one - reboot
I used the dirty method to change my username, and the right one to fix it again, so both are confirmed to work on 4.2