As far as steps:
- Backup
- apt update & full-upgrade
- Disable /etc/apt/sources.list.d/armbian.list all entries to stop armbian from upgrading
- Use normal debian upgrade steps -> Change release in /etc/apt/sources.list
- apt update & full-upgrade
- Restart, validate working system
- Update /etc/apt/sources.list.d/armbian.list with new release (buster->bullseye)
- apt update & full-upgrade
- Should be done.
Take extra care for some new/renamed packages. armbian-bsp-cli-<BOARD> is new. Older armbian-root-* packages dont exist anymore (I think).
For me this worked flawless, you might have to fix some dependency problems, and make sure at least some working version of armbian-image-* armbian-dtb-* is installed before rebooting.
