Answer
Most shops move from Power9 directly to Power10 or Power11 rather than upgrading generation by generation, since IBM i, RPG, COBOL, and Db2 for i all carry forward across Power generations without an application rewrite. The real project work is confirming your IBM i release is supported on the target generation, checking HMC version compatibility, and planning a tested data migration and cutover window.
Downtime is required for the actual cutover, but with proper planning that window can be scheduled during off-hours instead of forcing an unplanned outage. Going straight to Power11 is typically simpler than a two-step move through Power10 first, since it avoids migrating the same workload twice.