I haven’t seen a definitive assurance that crimson is ready to use for the Librem 15—though this one comes close—so I went ahead and made a full backup and dove in, using the Debian instructions as a model. (I’ve upgraded Debian systems dozens of times.)
It was very straightforward: change byzantium to crimson in /etc/apt/sources.list, then:
apt update
apt upgrade --without-new-pkgs
apt full-upgrade
Everything’s new and fresh now. I actually reduced disk usage by 6GB after cleaning up like the Debian instructions suggest.