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.