- Remember to use screen or tmux to avoid problems when the network connection breaks temporarily.
- Make sure to run dispatch-conf (recommended) or etc-config (only if you're still using it) carefully and completely (update all files). If in doubt, just run it again, it will exit immediately if there's nothing to update.
- Read the OpenRC migration guide. Don't glance over it, make sure to read every sentence carefully.
- Before doing the update in production, make sure to test the upgrade procedure in a non-production environment.
Now here's one thing that may be easy to miss: your /etc/conf.d/net file will most likely require a manual update. Everything is described in the migration guide, the change is trivial. I'd rather not experiment what happens if you don't do that, but there is a real chance the system will just not bring up the network interfaces. Similarly, make sure that your net.eth0 etc services exist (ls -l /etc/init.d/net*) and will be started on boot (rc-status).
That should be it. Please report any problems to the Gentoo Bugzilla.