My understanding is that slowroll is just tumbleweed with more stringent release criteria. Looks like the Agama installer for it might be missing systemdboot, but since you are migrating from Tumbleweed you won't be using that (its just a repo switch rather than a new install). So you should be good to migrate right now.
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Ah, so all it does is point me to different endpoints for Zypper? The migration doesn't change the configuration of the system otherwise?
One more thing that is not clear to me -- I ran zypper dup yesterday, my packages may be now ahead of Slowroll by a few weeks, does migrating mean I am downgrading or do my current packages stay as is and will be in sync with Slowroll later?
Righ, slowroll is just a different cadence of package versions, and it just switches to slowroll repos. As for whether the packages will downgrade, I would guess yes. This is because Slowroll takes the same approach as Tumbleweed, where each release is a snapshot of a configuration consisting of packages of a specific version that are known (to the best of the maintainers' ability) to work together. So they will likely downgrade to match the known good configuration.
Unless you crossed a major new feature release in Tumbleweed that hasn't made it to slowroll, I'd say that's probably not an issue.
I was upgraded to GNOME 50 and had systemd-boot from Tumbleweed installer. I think those two are major ones, the rest were just regular package updates. Well, including the kernel, but I suppose it's not an issue.