Quite an old thread without any reaction to it, but I myself attempted a
migration from buster to beowulf today, and thought about sharing my
experience.

> What is the current recommended way to crossgrade from buster to
> beowulf (so I can test it properly)

I followed
https://devuan.org/os/documentation/dev1fanboy/migrate-to-ascii which
happened to roughly do the trick.

To grab the Devuan keyring, temporarily use ascii repository.

> And what logging would be useful to do while I do this?

I suppose you could record your session the way Debian recommends it:
https://www.debian.org/releases/stable/amd64/release-notes/ch-upgrading.html#record-session

> In particular, is there any useful way to get a log during reboot to
> the new system?

On top of the session recording from above, I guess digging into
/var/log/dmesg, /var/log/messages (which might include the former) and
any other resource deemed useful.
I got myself some errors on boot I was not able to find in the logs
(probably happened before filesystem was mounted)

I am now having to deal with ugly network interfaces name, and a couple
related quirks & bruises everywhere.
My case was particular, as having installed buster w/o network from the
full DVD (not netinst), I ended without a desktop GUI & a minimal
install... I don't know what went wrong with that official Debian installer.
I had to use buster, as the kernel used in stretch (and ascii) was too
old for my network hardware which ascii did not detect.

I happened to notice, for instance, that USB drives were not popping up
on plug in Xfce4 GUI... Might be an install quirk... or a software defect?

Overall, it seems to be working. The basics are there.

Bernard Rosset
https://rosset.net/