My company is relatively small. With probably 6 separate redis instances deployed in various places (k8s, bare metal, staging and prod environments) and dozens of (micro)services using them it's probably at least 40 hours (one person-week) to migrate everything at this point. Also there are things like documentation, legacy apps that keep working but nobody wants to spend time updating them, naming problems everywhere (renaming "redis" everywhere with zero downtime would be a huge pain), outdated documentation, possibly updates in CI, CD, and e2e tests, and probably more problems that ight become apparent in scale.

And we're honestly not large. For a mid size company, hundreds hours sound reasonable. For a big company the amount of work must be truly staggering.