Apologies in delayed response.
I checked with internal team on this classic related query, have you tested cloud services migration in Private Preview to make sure it meets the your application requirements? One option would be to utilize the ‘redeploy’ path for migrating the existing cloud services to ARM and during redeployment, use the target subscription in ARM.
The subscription move for cloud services (extended support) in ARM is in product teams roadmap but won't be available until after GA of the service itself.
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