Plan the one-way boundary of availability-set migration to Flexible scale sets

Source facts

Microsoft labels availability-set migration to Flexible scale sets as preview. Migration deallocates VMs and cannot be reversed back to an availability set after completion. Canceling an unfinished migration leaves already-moved VMs in the scale set; only the remaining VMs stay in the availability set. The portal migrates the whole set together, whereas CLI, PowerShell, and REST workflows permit individual VM moves. Once migration begins, the availability set is locked to migration, completion, or cancellation operations until the process finishes. Migrated VMs require an explicit start. Microsoft Learn.

Applicability

Evaluate this preview only through an approved trial. Check registration and the documented disk, public-IP, and load-balancer exclusions before selecting a target. Record whether the workload can tolerate the proposed interruption and whether its migration needs per-VM sequencing.

DSE recommendation

Write a per-VM migration and interruption plan that does not treat cancellation as reversal. Name the intended target and zone assignment, the point at which each VM may move, and the operator responsible for starting it afterward. Include an explicit decision for a partially migrated estate: continue, pause for investigation, or cancel the remaining moves. Do not promise a return path that the procedure does not provide.

Verification

In a permitted pilot, capture validation results and the membership of each VM before and after its move. Exercise the approved start and workload checks before moving the next participant. If testing cancellation, compare the resulting split membership with the written expectation. Retain the old availability-set record until every intended VM has been accounted for and the cleanup decision is approved.

Official references

Microsoft Learn: Migrate availability sets to Virtual Machine Scale Sets. Source reviewed September 9, 2026.