Userspace upgrades (Focal -> Jammy, Bullseye -> Bookworm) are neither tested nor supported. There is no difference in Debian or Ubuntu flavor in that matter.
Manuals about how to do that may work though if you freeze firmware packages. Otherwise distro-specific kernel will be installed and this will almost certain break boot process.
Confirmed working with a freshly built image. However at least one partition must exist on the NVMe. A completely blank NVMe won't work.
However I think armbian-install received some fixes that might not be included in official 23.02 images. So building own is mandatory (or copy the latest armbian-install script into at least).
Hm did you run apt update beforehand?
Download works for me
apt-get download linux-headers-current-sunxi
Get:1 http://fi.mirror.armbian.de/apt buster/main armhf linux-headers-current-sunxi armhf 23.02.2 [12.0 MB]
Fetched 12.0 MB in 4s (3,045 kB/s)
You do not specifically have to use numpad to enter 1234. The other numbers will work too.
Alternatively you can boot it up with debug serial connected with will do auto-login on first boot.
Maybe it became too common knowledge that the following baud rates are most common:
Amlogic and Allwinner SoCs: 115200
Rockchip SoCs: 1500000
And yes, this could be documented somewhere if it isn't already on docs.armbian.com